Leetcode 20 Solution

This article provides solution to leetcode question 20 (valid-parentheses)

https://leetcode.com/problems/valid-parentheses

Thinking Process

This is a very classic question. Use stack to check the validness of parenthesis.

Time & Space Complexity

Assuming N is the length of the string

  • Time complexity: O(N)
  • Space complexity: O(1)

Solution

class Solution { public: bool isValid(string s) { stack<char> a;
for (int i = 0; i < s.length(); i++) { char ch = s[i]; if (ch == '(' || ch == '[' || ch == '{') a.push(ch); else if (ch == ')' || ch == ']' || ch == '}') { if (a.empty()) return false;
char ch2 = a.top(); a.pop();
if (ch == ')' && ch2 != '(') return false; else if (ch == ']' && ch2 != '[') return false; else if (ch == '}' && ch2 != '{') return false; } }
return a.empty(); } };