20. Valid Parentheses

• Difficulty: Easy

• Topics: String, Stack

• Similar Questions:

  • Generate Parentheses

  • Longest Valid Parentheses

  • Remove Invalid Parentheses

  • Check If Word Is Valid After Substitutions

Problem:

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

Solutions:

class Solution {
public:
    bool isValid(string s) {
        stack<char> stk;
        for (auto c : s) {
            switch(c) {
                case '(':
                case '{':
                case '[':
                    stk.push(c);
                    break;
                case ')':
                    if (stk.empty() || stk.top() != '(') return false;
                    stk.pop();
                    break;
                case '}':
                    if (stk.empty() || stk.top() != '{') return false;
                    stk.pop();
                    break;
                case ']':
                    if (stk.empty() || stk.top() != '[') return false;
                    stk.pop();
                    break;
                default:
                    continue;
            }
        }

        return stk.empty();
    }
};