[Swift]LeetCode1003. 檢查替換後的詞是否有效 | Check If Word Is Valid After Substitutions

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We are given that the string "abc" is valid.

From any valid string V, we may split V into two pieces X and Y such that X + Y (X concatenated with Y) is equal to V.  (X or Y may be empty.)  Then, X + "abc" + Y is also valid.

If for example S = "abc", then examples of valid strings are: "abc", "aabcbc", "abcabc", "abcabcababcc".  Examples of invalid strings are: "abccba", "ab", "cababc", "bac".

Return true if and only if the given string S is valid. 

Example 1:

Input: "aabcbc"
Output: true Explanation: We start with the valid string "abc". Then we can insert another "abc" between "a" and "bc", resulting in "a" + "abc" + "bc" which is "aabcbc". 

Example 2:

Input: "abcabcababcc"
Output: true Explanation: "abcabcabc" is valid after consecutive insertings of "abc". Then we can insert "abc" before the last letter, resulting in "abcabcab" + "abc" + "c" which is "abcabcababcc". 

Example 3:

Input: "abccba"
Output: false 

Example 4:

Input: "cababc"
Output: false 

Note:

1. 1 <= S.length <= 20000
2. S[i] is 'a', 'b', or 'c'

---

給定有效字符串 "abc"。

對於任何有效的字符串 V，咱們能夠將 V 分紅兩個部分 X 和 Y，使得 X + Y（X 與 Y 鏈接）等於 V。（X 或 Y 能夠爲空。）那麼，X + "abc" + Y 也一樣是有效的。

例如，若是 S = "abc"，則有效字符串的示例是："abc"，"aabcbc"，"abcabc"，"abcabcababcc"。無效字符串的示例是："abccba"，"ab"，"cababc"，"bac"。

若是給定字符串 S 有效，則返回 true；不然，返回 false。 

示例 1：

輸入："aabcbc"
輸出：true
解釋：
從有效字符串 "abc" 開始。
而後咱們能夠在 "a" 和 "bc" 之間插入另外一個 "abc"，產生 "a" + "abc" + "bc"，即 "aabcbc"。

示例 2：

輸入："abcabcababcc"
輸出：true
解釋：
"abcabcabc" 是有效的，它能夠視做在原串後連續插入 "abc"。
而後咱們能夠在最後一個字母以前插入 "abc"，產生 "abcabcab" + "abc" + "c"，即 "abcabcababcc"。

示例 3：

輸入："abccba"
輸出：false

示例 4：

輸入："cababc"
輸出：false 

提示：

1. 1 <= S.length <= 20000
2. S[i] 爲 'a'、'b'、或 'c'

---

44ms

 1 class Solution 
 2 {    
 3     func isValid( _ string: String ) -> Bool 
 4     {        
 5         var aStack: Int = 0
 6         var bStack: Int = 0      
 7         for char in string.characters
 8         {            
 9             if char == "a"
10             {
11                 aStack += 1
12             }
13             else if char == "b"
14             {                
15                 bStack += 1                
16                 if bStack > aStack
17                 {
18                     return false
19                 }                
20             }
21             else if char == "c"
22             {                
23                 aStack -= 1
24                 bStack -= 1                
25                 if aStack < 0 || bStack < 0
26                 {
27                     return false
28                 }                
29             }            
30         }        
31         return aStack == 0 && bStack == 0        
32     }    
33 }

---

52ms

 1 class Solution {
 2     func isValid(_ S: String) -> Bool {
 3         var stack = [Character]()
 4         for char in S {
 5             if char == "c" {
 6                 guard stack.count >= 2 else { return false }
 7                 guard stack.removeLast() == "b" else { return false }
 8                 guard stack.removeLast() == "a" else { return false }
 9             } else {
10                 stack.append(char)
11             }
12         }
13         return stack.count == 0
14     }
15 }

---

56ms

 1 class Solution {
 2     func isValid(_ S: String) -> Bool {
 3         var stack: [Character] = []
 4         for s in S {
 5             switch s {
 6             case "a", "b": stack.append(s)
 7             case "c":
 8                 guard stack.count >= 2,
 9                     stack.removeLast() == "b",
10                     stack.removeLast() == "a" else {
11                     return false
12                 }
13             default: fatalError()
14             }
15         }
16         
17         return stack.isEmpty
18     }
19 }

---

80ms

 1 class Solution {
 2     func isValid(_ S: String) -> Bool {
 3         let s = Array(S)
 4         var v = [Character]()
 5         var j = 0
 6         while j < s.count {
 7             v.append(s[j])
 8             if v.count >= 3 && 
 9                 v[v.count - 3] == "a" &&
10                 v[v.count - 2] == "b" &&
11                 v[v.count - 1] == "c" {
12                 v.removeLast(3)
13             }
14             j += 1
15         }
16         return v.count == 0
17     }
18 }

---

288ms

1 class Solution {
2     func isValid(_ S: String) -> Bool {
3         var S = S
4         while S.contains("abc") {
5             S = S.replacingOccurrences(of: "abc", with: "", options: .literal, range: nil)
6         }
7         return S.isEmpty
8     }
9 }

---

292ms

 1 class Solution {
 2     func isValid(_ S: String) -> Bool {
 3         var curS = S
 4         while curS.count >= 3 {
 5             let str = (curS as NSString).replacingOccurrences(of: "abc", with: "") 
 6                                                               
 7             curS = str       
 8             if !(str as NSString).contains("abc") { break}
 9         }
10         return curS.count == 0
11     }
12 }