Leetcode20

Valid Parentheses:
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: true

Solution:

#include <iostream>
#include <stack>
#include <string>
#include <cassert>

using namespace std;

class Solution {
public:
    bool isValid(string s) {

        stack<char> stack;

        for (int i = 0; i < s.size(); i++) {
            if( s[i] == '(' || s[i] == '[' || s[i] == '{')
                stack.push( s[i] );
            else{

                if( stack.size() == 0) // 这一步就能判断了（s[0] == ')' || s[0] == ']' || s[0] == '}'）的情况
                    return false;

                char c = stack.top();
                stack.pop();

                char match;
                if( s[i] == ')')
                    match = '(';
                else if ( s[i] == ']')
                    match = '[';
                else{
                    assert( s[i] == '}');
                    match = '{';
                }

                if( c != match )
                    return false;
            }
        }
        if( stack.size() != 0 )
            return false;
        return true;
    }
};

总结： 最基本有关栈的题，括号匹配问题。