Leetcode 20 Valid Parentheses Java Solution

// SOLVING THIS WITH AN AI ASSISTANT (2026)

If you are working through this problem with an AI coding assistant — Claude, ChatGPT, Cursor chat, Gemini, GitHub Copilot, Aider, or any agent — the goal isn’t to ask for the answer. It is to use the tool to understand the pattern. The prompt sequence I’d run:

Spec it back to me first. “In your own words, what is this problem actually testing? What’s the smallest example that fails the naive approach?”
Brute-force first, optimize after. “Write the simplest correct solution, even if it’s O(n²). Don’t optimize. Just make it correct, with comments explaining each step.”
Ask for the upgrade. “Now show me the optimal solution. What insight makes it possible? What pattern is this an instance of?”
Stress-test it. “Generate 10 edge cases — empty input, single element, duplicates, max size, sorted, reverse-sorted. Run my solution against each.”

The pattern matters more than the answer. If the agent just hands you optimized code, you’ve trained yourself to lose interviews.

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

Open brackets must be closed by the same type of brackets.
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

Solution 1:

class Solution {
    public boolean isValid(String s) {
        if(s.length()<=1) return false;
        Stack<Character> paraS = new Stack<Character>();
        for(int i=0;i<s.length();i++){
            if(s.charAt(i)=='('||s.charAt(i)=='{'||s.charAt(i)=='['){
                paraS.push(s.charAt(i));
            }else if(s.charAt(i)==')'){
                if(paraS.size()==0 || paraS.pop()!= '(') return false;
            }else if(s.charAt(i)==']'){
                if(paraS.size()==0 || paraS.pop()!= '[') return false;
            }else if(s.charAt(i)=='}'){
                if(paraS.size()==0 || paraS.pop()!= '{') return false;
            }
        }
        
        return paraS.isEmpty();
    }
}

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rajendra