Leetcode 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.

1 - 考查栈的基本应用
2 - std::string::npos
static const size_t npos = -1;
Maximum value for size_t
npos is a static member constant value with the greatest possible value for an element of type size_t.

This value, when used as the value for a len (or sublen) parameter in string’s member functions, means “until the end of the string”.

As a return value, it is usually used to indicate no matches.

This constant is defined with a value of -1, which because size_t is an unsigned integral type, it is the largest possible representable value for this type.

class Solution {public:    bool isValid(string s) {        string left = "({[";        string right = ")}]";        stack<char> stk;        for(char c : s){            //若字符为左括号，则入栈            if(left.find(c)!=string::npos){                stk.push(c);            //若字符为右括号时，栈空或者和左括号无法匹配，则返回false            }else if(stk.empty()||left[right.find(c)]!=stk.top()){                return false;            //否则左括号出栈            }else{                stk.pop();            }        }        if(stk.empty()) return true;        else return false;    }};