[Swift]LeetCode1003. 检查替换后的词是否有效 | 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'

---

给定有效字符串 "abc"。

对于任何有效的字符串 V，我们可以将 V 分成两个部分 X 和 Y，使得 X + Y（X 与 Y 连接）等于 V。（X 或 Y 可以为空。）那么，X + "abc" + Y 也同样是有效的。

例如，如果 S = "abc"，则有效字符串的示例是："abc"，"aabcbc"，"abcabc"，"abcabcababcc"。无效字符串的示例是："abccba"，"ab"，"cababc"，"bac"。

如果给定字符串 S 有效，则返回 true；否则，返回 false。 

示例 1：

输入："aabcbc"
输出：true
解释：
从有效字符串 "abc" 开始。
然后我们可以在 "a" 和 "bc" 之间插入另一个 "abc"，产生 "a" + "abc" + "bc"，即 "aabcbc"。

示例 2：

输入："abcabcababcc"
输出：true
解释：
"abcabcabc" 是有效的，它可以视作在原串后连续插入 "abc"。
然后我们可以在最后一个字母之前插入 "abc"，产生 "abcabcab" + "abc" + "c"，即 "abcabcababcc"。

示例 3：

输入："abccba"
输出：false

示例 4：

输入："cababc"
输出：false 

提示：

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

---

44ms

class Solution 
{    
    func isValid( _ string: String ) -> Bool 
    {        
        var aStack: Int = 0
        var bStack: Int = 0      
        for char in string.characters
        {            
            if char == "a"
            {
                aStack += 1
            }
            else if char == "b"
            {                
                bStack += 1                
                if bStack > aStack
                {
                    return false
                }                
            }
            else if char == "c"
            {                
                aStack -= 1
                bStack -= 1                
                if aStack < 0 || bStack < 0
                {
                    return false
                }                
            }            
        }        
        return aStack == 0 && bStack == 0        
    }    
}

---

52ms

class Solution {
    func isValid(_ S: String) -> Bool {
        var stack = [Character]()
        for char in S {
            if char == "c" {
                guard stack.count >= 2 else { return false }
                guard stack.removeLast() == "b" else { return false }
                guard stack.removeLast() == "a" else { return false }
            } else {
                stack.append(char)
            }
        }
        return stack.count == 0
    }
}

---

56ms

class Solution {
    func isValid(_ S: String) -> Bool {
        var stack: [Character] = []
        for s in S {
            switch s {
            case "a", "b": stack.append(s)
            case "c":
                guard stack.count >= 2,
                    stack.removeLast() == "b",
                    stack.removeLast() == "a" else {
                    return false
                }
            default: fatalError()
            }
        }
        
        return stack.isEmpty
    }
}

---

80ms

class Solution {
    func isValid(_ S: String) -> Bool {
        let s = Array(S)
        var v = [Character]()
        var j = 0
        while j < s.count {
            v.append(s[j])
            if v.count >= 3 && 
                v[v.count - 3] == "a" &&
                v[v.count - 2] == "b" &&
                v[v.count - 1] == "c" {
                v.removeLast(3)
            }
            j += 1
        }
        return v.count == 0
    }
}

---

288ms

class Solution {
    func isValid(_ S: String) -> Bool {
        var S = S
        while S.contains("abc") {
            S = S.replacingOccurrences(of: "abc", with: "", options: .literal, range: nil)
        }
        return S.isEmpty
    }
}

---

292ms

class Solution {
    func isValid(_ S: String) -> Bool {
        var curS = S
        while curS.count >= 3 {
            let str = (curS as NSString).replacingOccurrences(of: "abc", with: "") 
                                                              
            curS = str       
            if !(str as NSString).contains("abc") { break}
        }
        return curS.count == 0
    }
}