[leetcode]Valid Parentheses

简单题。使用stack就行了。不过一开始忘了判断'['和']'的情况。要判断stack是否为空。

#include <string>
#include <stack>

using namespace std;

class Solution {
public:
    bool isValid(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        stack<char> st;
    	for (int i = 0; i < s.size(); i++)
		{
			if (s[i] == '(' || s[i] == '[' || s[i] == '{')
			{
				st.push(s[i]);
			}
			else if (s[i] == ')' || s[i] == ']' || s[i] == '}')
			{
                if (st.empty()) return false;
				char c = st.top();
				st.pop();
				if (c == '(' && s[i] != ')') return false;
				if (c == '[' && s[i] != ']') return false;
				if (c == '{' && s[i] != '}') return false;
			}
		}
        if (!st.empty()) return false;
		return true;
    }
};

python3

class Solution:
    def isValid(self, s: str) -> bool:
        stack = []
        mapping = {'(': ')', '{': '}', '[': ']'}
        for c in s:
            if len(stack) == 0:
                stack.append(c)
            else:
                p = stack.pop()
                if p in mapping and c == mapping[p]:
                    continue
                else:
                    stack.append(p)
                    stack.append(c)
        return len(stack) == 0