20 Valid Parentheses

Given a string containing just the characters ‘(‘, ‘)‘, ‘{‘, ‘}‘, ‘[‘ and ‘]‘, determine if the input string is valid.

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

自己代碼：

class Solution {
public:
    bool isValid(string
 
 s) {
        stack<char> parent;
        for(char c:s){
            if(c ==‘[‘||c == ‘(‘ || c == ‘{‘) parent.push(c);  //左括號進棧
            else if(parent.empty()) return false;   
            else{                                              //有右括號且與棧頂匹配，則棧頂元素出棧
                if(c ==‘]‘ && parent.top() == ‘
 
[‘) parent.pop();
                else if(c ==‘}‘ && parent.top() == ‘{‘) parent.pop();
                else if(c ==‘)‘ && parent.top() == ‘(‘) parent.pop();
                else return false;
            }
        }
        if(parent.empty()) return true;
        else return false;
    }
    
};
 

巧妙的方法，discussion區發現：

遇到左括號，則使對應右括號進棧，例如：遇到“{”，進棧“}”。遇到“[”，進棧“]”。遇到“(”，進棧“)”。

非右括號，則看它是否與棧頂元素相等，相等即匹配。

public boolean isValid(String s) {
    Stack<Character> stack = new Stack<Character>();
    for (char c : s.toCharArray()) {
        if (c == ‘(‘)                                 //遇到左右括號，使括號進棧
            stack.push(‘)‘);
        else if (c == ‘{‘)
            stack.push(‘}‘);
        else if (c == ‘[‘)
            stack.push(‘]‘);
        else if (stack.isEmpty() || stack.pop() != c)  //遇到非右括號，若棧空，返回false。否則彈出棧頂元素，看其是否與當前元素相等，否則錯誤
            return false;
    }
    return stack.isEmpty();
}

用switch語句寫：

 bool isValid(string s) {
        stack<char> paren;
        for (char& c : s) {
            switch (c) {
                case ‘(‘: 
                case ‘{‘: 
                case ‘[‘: paren.push(c); break;
                case ‘)‘: if (paren.empty() || paren.top()!=‘(‘) return false; else paren.pop(); break;
                case ‘}‘: if (paren.empty() || paren.top()!=‘{‘) return false; else paren.pop(); break;
                case ‘]‘: if (paren.empty() || paren.top()!=‘[‘) return false; else paren.pop(); break;
                default: ; // pass
            }
        }
        return paren.empty() ;
    }

20. Valid Parentheses括號匹配