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

An input string is valid if:

• Open brackets must be closed by the same type of brackets.
• Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example 1：

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "([)]"
Output: false

解題思路

使用棧來儲存讀入的每一個括號字元，若遇到棧頂與下一個字元是否匹配，若匹配則彈出棧頂元素，否則將下一個字元入棧，遍歷字元結束後看棧內是否還有元素，若沒有則是正確，否則錯誤

Solution

class Solution {
public:
    bool isValid(string s) {
        int size = s.length();
        if(size == 0) return true;
        stack<char> container;
        container.push(s[0]);
        for(int i = 1; i < size; ++i) {
            if(!container.empty()) {
                if(container
 
.top() == '(' && s[i] == ')') {
                    container.pop();
                }else if(container.top() == '{' && s[i] == '}'){
                    container.pop();
                }else if(container.top() == '[' && s[i] == ']') {
                    container.pop();
                }else {
                    container.push(s[i]);
                }
            }else {
                container.push(s[i]);
            }
        }
        if(container.empty()) {
            return true;
        }else {
            return false;
        }
    }
};