20. Valid Parentheses

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20. Valid Parentheses

Description

Difficulty: Easy

Related Topics: String, Stack

Given a string s 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.
  3. Every close bracket has a corresponding open bracket of the same type.

Example 1:

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Input: s = "()"
Output: true

Example 2:

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Input: s = "()[]{}"
Output: true

Example 3:

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Input: s = "(]"
Output: false

Constraints:

  • 1 <= s.length <= 104
  • s consists of parentheses only '()[]{}'.

Hints/Notes

Solution

Language: C++

Cleaner solution:

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