LeetCode-in-All

20. Valid Parentheses

Easy

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.

Example 1:

Input: s = “()”

Output: true

Example 2:

Input: s = “()[]{}”

Output: true

Example 3:

Input: s = “(]”

Output: false

Example 4:

Input: s = “([)]”

Output: false

Example 5:

Input: s = “{[]}”

Output: true

Constraints:

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

Solution

class Solution {
  bool isValid(String s) {
    List<String> stack = [];
    for (int i = 0; i < s.length; i++) {
      String c = s[i];

      if (c == '(' || c == '[' || c == '{') {
        stack.add(c);
      } else if (c == ')' && stack.isNotEmpty && stack.last == '(') {
        stack.removeLast();
      } else if (c == '}' && stack.isNotEmpty && stack.last == '{') {
        stack.removeLast();
      } else if (c == ']' && stack.isNotEmpty && stack.last == '[') {
        stack.removeLast();
      } else {
        return false;
      }
    }

    return stack.isEmpty;
  }
}

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