0020. Valid Parentheses

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

Description

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 '()[]{}'.

ac

class Solution {
    public boolean isValid(String s) {
        // edge cases
        if (s == null || s.length() <= 1) return false;

        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == '(' || c == '{' || c == '[') {
                stack.push(c);
            } else {
                if (stack.isEmpty()) return false;
                char p = stack.pop();
                if (c == ')' && p == '(' || c == '}' && p == '{' || c == ']' && p == '[') {
                    continue;
                } else {
                    return false;
                }
            }
        }

        return stack.isEmpty();
    }
}