[LeetCode] 20. Valid Parentheses

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

Thought process:
Iterate through the string:
  1. If the character is an opening parenthesis, push it onto a stack.
  2. If the character is a closing parenthesis, compare it with the parenthesis on top of the stack:
    1. If they match, pop the top parenthesis off.
    2. If they don't match, return false.
Return if the stack is empty at the end of loop.

Solution:

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class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<>();
        for (char c : s.toCharArray()) {
            if (c == '(' || c == '[' || c == '{') {
                stack.push(c);
            } else if (stack.isEmpty()) {
                return false;
            } else if (c == ')' && stack.pop() != '(') {
                return false;
            } else if (c == ']' && stack.pop() != '[') {
                return false;
            } else if (c == '}' && stack.pop() != '{') {
                return false;
            }
        }
        return stack.isEmpty();
    }
}

Time complexity: O(n).
Location: San Jose, CA, USA

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