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

 1 class Solution {
 2     public char map(char c) {
 3         if (c == ')') return '(';
 4         if (c == ']') return '[';
 5         if (c == '}') return '{';
 6         return ' ';
 7     }
 8     public boolean isValid(String s) {
 9         int n = s.length();
10         Map<String, String> map = new HashMap<>();
11         
12         if (n == 0) return true;
13         char []stack = new char[n];
14         int k = 0;
15         for (int i = 0; i < n; ++i) {
16             char c = s.charAt(i);
17             if (c == '(' || c == '[' || c == '{') {
18                 stack[k++] = c;
19             } else {
20                 if (k == 0 || stack[--k] != map(c)) {
21                     return false;
22                 }
23             }
24         }
25         if (k== 0)
26             return true;
27         else 
28             return false;
29     }
30 }

 

posted @ 2020-02-13 19:58  hyx1  阅读(126)  评论(0)  编辑  收藏  举报