數據結構與算法JavaScript (四) 串(BF)

時間 2019-11-19

標籤數據結構算法 javascript 欄目 JavaScript 简体版

原文原文鏈接

串是由零個或多個字符組成的有限序列，又叫作字符串javascript

串的邏輯結構和線性表很類似的，不一樣的是串針對是是字符集，因此在操做上與線性表仍是有很大區別的。線性表更關注的是單個元素的操做CURD，串則是關注查找子串的位置，替換等操做。html

固然不一樣的高級語言對串的基本操做都有不一樣的定義方法，可是總的來講操做的本質都是類似的。好比javascrript查找就是indexOf, 去空白就是trim,轉化大小寫toLowerCase/toUpperCase等等java

這裏主要討論下字符串模式匹配的幾種經典的算法：BF、BM、KMPgit

BF(Brute Force)算法github

Brute-Force算法的基本思想：算法

從目標串s 的第一個字符起和模式串t的第一個字符進行比較，若相等，則繼續逐個比較後續字符，不然從串s 的第二個字符起再從新和串t進行比較。express

依此類推，直至串t 中的每一個字符依次和串s的一個連續的字符序列相等，則稱模式匹配成功，此時串t的第一個字符在串s 中的位置就是t 在s中的位置，不然模式匹配不成功app

可見BF算法是一種暴力算法，又稱爲樸素匹配算法或蠻力算法。ide

主串 BBC ABB ABCF性能

子串 ABC

在主串中找出子串的位置，對應了其實就是javascript的indexOf查找方法的實現了

var sourceStr = "BBC ABB ABCF";
var searchStr = "ABC";

function BF_Ordinary(sourceStr, searchStr) {
  var sourceLength = sourceStr.length;
  var searchLength = searchStr.length;
  var padding      = sourceLength - searchLength; //循環的次數
  //BBC ABB ABCF =>ABC => 搜索9次
  for (var i = 0; i <= padding; i++) {
    //若是知足了第一個charAt是相等的
    //開始子循環檢測
    //其中sourceStr的取值是須要疊加i的值
    if (sourceStr.charAt(i) == searchStr.charAt(0)) {
      //匹配成功的數據
      var complete = searchLength;
      for (var j = 0; j < searchLength; j++) {
        if (sourceStr.charAt(i + j) == searchStr.charAt(j)) {
          --complete
          if (!complete) {
            return i;
          }
        }
      }
    }
  }
  return -1;
}

BF算法就是簡單粗暴，直接把BBC ABB ABCF母串的每個字符的下表取出來與模式串的第一個字符匹配，若是相等就進去字串的再次匹配

這裏值得注意：

1：最外圍循環的次數sourceLength - searchLength，由於咱們匹配的母串至少要大於等於子串

2：在子串的繼續匹配中，母串的起點是須要疊加的(i+j)

3：經過一個條件判斷是否徹底匹配complete，BBC ABB ABCF中，咱們在ABB的時候就須要跳過去

上面是最簡單的一個算法了，代碼上還有更優的處理，好比在自串的匹配上能夠採起取反的算法

優化算法（一）

function BF_Optimize(sourceStr, searchStr) {
    var mainLength   = sourceStr.length;
    var searchLength = searchStr.length;
    var padding      = mainLength - searchLength;
    for (var offset = 0; offset <= padding; offset++) {
      var match = true;
      for (var i = 0; i < searchLength; i++) {
        //取反,若是隻要不相等
        if (searchStr.charAt(i) !== sourceStr.charAt(offset + i)) {
          match = false;
          break;
        }
      }
      if (match) return offset;
    }
    return -1;
}

咱們不須要判斷爲真的狀況，咱們只要判斷爲假的狀況就能夠了，當子匹配結束後match沒有被修改過的話，則說明此匹配是徹底匹配

以上2種方法咱們都用到了子循環，咱們可否改爲一個循環體呢？

其實咱們能夠看到規律，主串每次都只會遞增+1，子串每次匹配也是從頭開始匹配，因此咱們能夠改爲一個while，控制下標指針就能夠了

優化算法（二）

function BF_Optimize_2(sourceStr, searchStr) {
  var i = 0,
      j = 0;

    while (i < sourceStr.length) {
        // 兩字母相等則繼續  
        if (sourceStr.charAt(i) == searchStr.charAt(j)) {
          i++;
          j++;
        } else { // 兩字母不等則角標後退從新開始匹配  
          i = i - j + 1; // i 回退到上次匹配首位的下一位  
          j = 0; // j 回退到子串的首位  
        }

        if (j == searchStr.length) {
          return i - j;
        }

    }
}

i就是主串的下標定位，j就是子串的下標定位

當主串子串相等的時候，就進入了子串的循環模式，當子循環的次數j知足子串長度時，就驗證是徹底匹配

當主串子串不相等的時候，就須要把主串的下標日後移一位，固然i的時候，由於可能通過子串的處理，因此須要i-j+1, 而後復位子串

具體咱們能夠看看代碼比較

基於BF算法的四種結構,for/while/遞歸

<!doctype html>因爲電腦性能的不斷提升，測試的數據量的大小，可能會致使獲得的結果不太準確;<script type="text/javascript"> ///////// //暴力算法 // //普通版 ///////// function BF_Ordinary(sourceStr, searchStr) { var sourceLength = sourceStr.length; var searchLength = searchStr.length; var padding = sourceLength - searchLength; //循環的次數 //BBC ABB ABCF =>ABC => 搜索9次 for (var i = 0; i <= padding; i++) { //若是知足了第一個charAt是相等的 //開始子循環檢測 //其中sourceStr的取值是須要疊加i的值 if (sourceStr.charAt(i) == searchStr.charAt(0)) { //匹配成功的數據 var complete = searchLength; for (var j = 0; j < searchLength; j++) { if (sourceStr.charAt(i + j) == searchStr.charAt(j)) { --complete if (!complete) { return i; } } } } } return -1; } ///////// //暴力算法 // //優化版 ///////// function BF_Optimize_1(sourceStr, searchStr) { var mainLength = sourceStr.length; var searchLength = searchStr.length; var padding = mainLength - searchLength; for (var offset = 0; offset <= padding; offset++) { var match = true; for (var i = 0; i < searchLength; i++) { //取反,若是隻要不相等 if (searchStr.charAt(i) !== sourceStr.charAt(offset + i)) { match = false; break; } } if (match) return offset; } return -1; } //////// //優化版 // //while //////// function BF_Optimize_2(sourceStr, searchStr) { var i = 0, j = 0; while (i < sourceStr.length) { // 兩字母相等則繼續 if (sourceStr.charAt(i) == searchStr.charAt(j)) { i++; j++; } else { // 兩字母不等則角標後退從新開始匹配 i = i - j + 1; // i 回退到上次匹配首位的下一位 j = 0; // j 回退到子串的首位 } if (j == searchStr.length) { return i - j; } } } ///////// //暴力算法 //遞歸版本 ///////// function BF_Recursive(sourceStr, searchStr, offset) { var mainLength = sourceStr.length; var searchLength = searchStr.length; if (searchLength > mainLength - offset) { return -1; } offset = offset || 0; for (var i = 0; searchLength > i; i++) { if (searchStr.charAt(i) !== sourceStr.charAt(offset + i)) { return BF_Recursive(sourceStr, searchStr, offset + 1) } } return offset; } var sourceStr = "There are some times wThere are some times when clicking 「like」 on a friend’s Facebook status doesn’t feel appropriate. A bad day. A loved one lost. A break up. It only seems natural that a 「dislike」 button could solve the conundrum of wanting to empathize but not seem inappropriate by clicking 「like.」 Mark Zuckerberg Puts the Rest of Us to Shame by Speaking Fluent Chinese. Mark Zuckerberg: Facebook Founder and Animal Butcher. Mark Zuckerberg and That Shirt. The idea has been on Mark Zuckerberg’s radar for a while, he said. In 2010, he told ABC News’ Diane Sawyer that that Facebook would 「definitely thinkThere are some times when clicking 「like」 on a friend’s Facebook status doesn’t feel appropriate. A bad day. A loved one lost. A break up. It only seems natural that a 「dislike」 button could solve the conundrum of wanting to empathize but not seem inappropriate by clicking 「like.」 Mark Zuckerberg Puts the Rest of Us to Shame by Speaking Fluent Chinese. Mark Zuckerberg: Facebook Founder and Animal Butcher. Mark Zuckerberg and That Shirt. The idea has been on Mark Zuckerberg’s radar for a while, he said. In 2010, he told ABC News’ Diane Sawyer that that Facebook would 「definitely thinkThere are some times when clicking 「like」 on a friend’s Facebook status doesn’t feel appropriate. A bad day. A loved one lost. A break up. It only seems natural that a 「dislike」 button could solve the conundrum of wanting to empathize but not seem inappropriate by clicking 「like.」 Mark Zuckerberg Puts the Rest of Us to Shame by Speaking Fluent Chinese. Mark Zuckerberg: Facebook Founder and Animal Butcher. Mark Zuckerberg and That Shirt. The idea has been on Mark Zuckerberg’s radar for a while, he said. In 2010, he told ABC News’ Diane Sawyer that that Facebook would 「definitely thinkThere are some times when clicking 「like」 on a friend’s Facebook status doesn’t feel appropriate. A bad day. A loved one lost. A break up. It only seems natural that a 「dislike」 button could solve the conundrum of wanting to empathize but not seem inappropriate by clicking 「like.」 Mark Zuckerberg Puts the Rest of Us to Shame by Speaking Fluent Chinese. Mark Zuckerberg: Facebook Founder and Animal Butcher. Mark Zuckerberg and That Shirt. The idea has been on Mark Zuckerberg’s radar for a while, he said. In 2010, he told ABC News’ Diane Sawyer that that Facebook would 「definitely thinkThere are some times when clicking 「like」 on a friend’s Facebook status doesn’t feel appropriate. A bad day. A loved one lost. A break up. It only seems natural that a 「dislike」 button could solve the conundrum of wanting to empathize but not seem inappropriate by clicking 「like.」 Mark Zuckerberg Puts the Rest of Us to Shame by Speaking Fluent Chinese. Mark Zuckerberg: Facebook Founder and Animal Butcher. Mark Zuckerberg and That Shirt. The idea has been on Mark Zuckerberg’s radar for a while, he said. In 2010, he told ABC News’ Diane Sawyer that that Facebook would 「definitely thinkThere are some times when clicking 「like」 on a friend’s Facebook status doesn’t feel appropriate. A bad day. A loved one lost. A break up. It only seems natural that a 「dislike」 button could solve the conundrum of wanting to empathize but not seem inappropriate by clicking 「like.」 Mark Zuckerberg Puts the Rest of Us to Shame by Speaking Fluent Chinese. Mark Zuckerberg: Facebook Founder and Animal Butcher. Mark Zuckerberg and That Shirt. The idea has been on Mark Zuckerberg’s radar for a while, he said. In 2010, he told ABC News’ Diane Sawyer that that Facebook would 「definitely thinkhen clicking 「like」 on a friend’s Facebook status doesn’t feel appropriate. A bad day. A loved one lost. A break up. It only seems natural that a 「dislike」 button could solve the conundrum of wanting to empathize but not seem inappropriate by clicking 「like.」 Mark Zuckerberg Puts the Rest of Us to Shame by Speaking Fluent Chinese. Mark Zuckerberg: Facebook Founder and Animal Butcher. Mark Zuckerberg and That Shirt. The idea has been on Mark Zuckerberg’s radar for a while, he said. In 2010, he told ABC News’ Diane Sawyer There are some times when clicking 「like」 on a friend’s Facebook status doesn’t feel appropriate. A bad day. A loved one lost. A break up. It only seems natural that a 「dislike」 button could solve the conundrum of wanting to empathize but not seem inappropriate by clicking 「like.」 Mark Zuckerberg Puts the Rest of Us to Shame by Speaking Fluent Chinese. Mark Zuckerberg: Facebook Founder and Animal Butcher. Mark Zuckerberg and That Shirt. The idea has been on Mark Zuckerberg’s radar for a while, he said. In 2010, he told ABC News’ Diane Sawyer that that Facebook would 「definitely thinkThere are some times when clicking 「like」 on a friend’s Facebook status doesn’t feel appropriate. A bad day. A loved one lost. A break up. It only seems natural that a 「dislike」 button could solve the conundrum of wanting to empathize but not seem inappropriate by clicking 「like.」 Mark Zuckerberg Puts the Rest of Us to Shame by Speaking Fluent Chinese. Mark Zuckerberg: Facebook Founder and Animal Butcher. Mark Zuckerberg and That Shirt. The idea has been on Mark Zuckerberg’s radar for a while, he said. In 2010, he told ABC News’ Diane Sawyer that that Facebook would 「definitely thinkThere are some times when clicking 「like」 on a friend’s Facebook status doesn’t feel appropriate. A bad day. A loved one lost. A break up. It only seems natural that a 「dislike」 button could solve the conundrum of wanting to empathize but not seem inappropriate by clicking 「like.」 Mark Zuckerberg Puts the Rest of Us to Shame by Speaking Fluent Chinese. Mark Zuckerberg: Facebook Founder and Animal Butcher. Mark Zuckerberg and That Shirt. The idea has been on Mark Zuckerberg’s radar for a while, he said. In 2010, he told ABC News’ Diane Sawyer that that Facebook would 「definitely thinkThere are some times when clicking 「like」 on a friend’s Facebook status doesn’t feel appropriate. A bad day. A loved one lost. A break up. It only seems natural that a 「dislike」 button could solve the conundrum of wanting to empathize but not seem inappropriate by clicking 「like.」 Mark Zuckerberg Puts the Rest of Us to Shame by Speaking Fluent Chinese. Mark Zuckerberg: Facebook Founder and Animal Butcher. Mark Zuckerberg and That Shirt. The idea has been on Mark Zuckerberg’s radar for a while, he said. In 2010, he told ABC News’ Diane Sawyer that that Facebook would 「definitely think that that Facebook would 「definitely think about」 adding a dislike button. 「People definitely seem to want it,」 Zuckerberg said. Four years later — Zuckerberg says Facebook is still 「thinking about」 adding the oft-requested sdfafd button, Zuckerberg says Facebook is still 「thinking about」 adding the oft-requested button. At a town hall meeting on Thursday, the CEO revealed he has some reservations about the feature. 「There are two things that it can mean,」 Zuckerberg said of the potential button, which could be used in a mean spirited way or to express empathy. Finding how to limit it to the latter is the challenge. Zuckerberg said he doesn’t want the button to turn into a 「voting mechanism」 or something that isn’t 「socially valuable.」「Often people will tell us they don’t feel comfortable pressing ‘like,'」 Zuckerberg said. 「What’s the right way to make it so people can easier express a wide range of emotions?」 One suggestion percolating online: Aaron Roll out the feature under a different name. However, an 「empathy button」 just may not have the same ring to it as 「dislike.」"; var searchStr = "adding the oft-requested sdf"; function show(bf_name,fn) { var myDate = +new Date() var r = fn(); var div = document.createElement('div') div.innerHTML = bf_name +'算法,搜索位置:' + r + ",耗時" + (+new Date() - myDate) + "ms"; document.body.appendChild(div); } show('BF_Ordinary',function() { return BF_Ordinary(sourceStr, searchStr) }) show('BF_Optimize_1',function() { return BF_Optimize_1(sourceStr, searchStr) }) show('BF_Optimize_2',function() { return BF_Optimize_2(sourceStr, searchStr) }) show('BF_Recursive',function() { return BF_Recursive(sourceStr, searchStr) }) </script>

BF也是經典的前綴匹配算法，前綴還包括KMP，咱們可見這種算法最大缺點就是字符匹配失敗指針就要回溯，因此性能很低，以後會寫一下KMP與BM算法針對BF的的升級

git代碼下載: https://github.com/JsAaron/data_structure