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:
 3     bool isValid(string s) {
 4         int l = s.length();
 5         if
 
(l == 0) {
 6             return true;
 7         }
 8         
 9         stack<char> st;
10         for(int i=0; i<l; i++) {
11             if(s[i] == ‘(‘ || s[i] == ‘[‘ || s[i] == ‘{‘) {
12                 st.push(s[i]);
13             }
14             else if(st.size() == 0) {
15                 return
 
 false;
16             }
17             else {
18                 if(s[i] == ‘)‘) {
19                     if(st.top() == ‘(‘) {
20                         st.pop();
21                     }
22                     else {
23                         return false;
24                     }
25                 }
26                 else if(s[i] == ‘]‘) {
27                     if(st.top() == ‘[‘) {
28                         st.pop();
29                     }
30                     else {
31                         return false;
32                     }
33                 }
34                 else {
35                     if(st.top() == ‘{‘) {
36                         st.pop();
37                     }
38                     else {
39                         return false;
40                     }
41                 }
42             }
43         }
44         if(st.size() == 0) {
45             return true;
46         }
47         else {
48             return false;
49         }
50     }
51 };

LeetCode - 20. Valid Parentheses(0ms)