LeetCode-20. Valid Parentheses

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

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. 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: true

题目：括号匹配，按照数学里的括号匹配规则即可。

思路：括号匹配问题是数据结构中栈必学习的内容，对于string s，如果是左括号，则入栈，如果是右括号，查看是否与栈顶成对，不成对则return false，成对则将栈顶元素出栈。遍历完s之后检查栈是否为空，不为空返回false,否则返回true。

代码：

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

AC beats100%。