20. Valid Parentheses

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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.

题意：判断给定的一个括号串是否合法。

思路：利用栈，遇见左括号入栈，遇见右括号，配对则出栈或否则则报错。简单题，而且以前肯定遇到过，一次AC吧~

class Solution {public:bool isValid(string s) {stack<char> stac;stac.push('$');int i = 0;while (i < s.size()){switch (stac.top()){case '$':if (s[i] == '(' || s[i] == '[' || s[i] == '{'){stac.push(s[i]);i++;}elsereturn false;break;case '(':if (s[i] == ')'){stac.pop();i++;}else if (s[i] == '(' || s[i] == '[' || s[i] == '{'){stac.push(s[i]);i++;}elsereturn false;break;case '[':if (s[i] == ']'){stac.pop();i++;}else if (s[i] == '(' || s[i] == '[' || s[i] == '{'){stac.push(s[i]);i++;}elsereturn false;break;case '{':if (s[i] == '}'){stac.pop();i++;}else if (s[i] == '(' || s[i] == '[' || s[i] == '{'){stac.push(s[i]);i++;}elsereturn false;break;default:return false;break;}}if (stac.top() == '$')return true;elsereturn false;}};

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