【LeetCode】415 Add Strings （java實現）

原題連接

https://leetcode.com/problems/add-strings/

原題

Given two non-negative numbers num1 and num2 represented as string, return the sum of num1 and num2.

Note:

The length of both num1 and num2 is < 5100.
Both num1 and num2 contains only digits 0-9.
Both num1 and num2 does not contain any leading zero.
You must not use any built-in BigInteger library or convert the inputs to integer directly.

題目要求

題目叫「字符串相加」，給定兩個非負的由字符串表示整數，求出它們的和。這裏須要注意的是，num1和num2字符串的長度都小於5100，這是個很大的數字，確定不能用int來計算。數字只包含0-9，也就是說沒有負號，沒有小數點等，徹底當作整形來計算。不容許使用內建的大整形的庫函數。

解法

實現一：思路比較簡單，既然不能把字符串轉爲整形來計算，那就一個字符一個字符取出來相加，算好進位便可。

public String addStrings(String num1, String num2) {
    String bigStr = null;
    String smallStr = null;
    
    if (num1.length() >= num2.length()) {
        bigStr = num1;
        smallStr = num2;
    } else {
        bigStr = num2;
        smallStr = num1;
    }
    
    int big = bigStr.length();
    int small = smallStr.length();
    int carry = 0;
    char []ra = new char[big + 1];
    for (int i = 0; i < small; i++) {
        int b = Character.getNumericValue(bigStr.charAt(big - i - 1));
        int s = Character.getNumericValue(smallStr.charAt(small - i - 1));
        ra[big - i] = Character.forDigit((b + s + carry) % 10, 10);
        carry = (b + s + carry) / 10; 
    }

    for (int i = 0; i < big - small; i++) {
        int b = Character.getNumericValue(bigStr.charAt(big - small - i - 1));
        ra[big - small - i] = Character.forDigit((b+ carry) % 10, 10);
        carry = (b + carry)/ 10; 
    }
    if (carry != 0) {
        ra[0] = Character.forDigit(carry % 10, 10);
    }
    
    String ret = new String(ra).trim();
    return ret;
}

實現二：從disguss選出來的最簡解，我把它從c++翻譯成了Java。與實現一併無什麼本質不一樣，只是代碼更簡潔，更清晰。牛人太多，佩服。

public String addStrings(String num1, String num2) {
    int i = num1.length() - 1, j = num2.length() - 1, carry = 0;
    String res = "";
    while (i >= 0 || j >= 0) {
        if (i >= 0)
            carry += num1.charAt(i--) - '0';
        if (j >= 0)
            carry += num2.charAt(j--) - '0';
        res = Integer.toString(carry % 10) + res;
        carry /= 10;
    }
    return carry != 0 ? "1" + res : res;
}