30 Days of LeetCode: Day 4 - Valid Parentheses String Locking Pattern

When I first encountered the "Check if a Parentheses String Can Be Valid" challenge, I fell into a common trap that many developers encounter: trying to solve for creation when the problem only asked for validation. This experience taught me a valuable lesson about problem interpretation and efficient solution design.

The Challenge

Given a string of parentheses and a corresponding "locked" pattern (where '1' means the character is locked and '0' means it can be changed), determine if it's possible to create a valid parentheses string.

Initial Approach

My first attempt focused on physically transforming the string into valid parentheses:

var canBeValid = function(s, locked) {
    if (s.length % 2 !== 0) return false;
    let arr = s.split('');

    for (let i = 0; i < arr.length; i += 2) {
        if (locked[i] === '0' && locked[i+1] === '0') {
            arr[i] = '(';
            arr[i+1] = ')';
        }
    }
    // Check validity...
};

This approach had several flaws:

1. Only looked at adjacent pairs

2. Tried to physically create the valid string

3. Assumed pairs had to be adjacent

The Breakthrough

The "aha" moment came when I realized: We don't need to create a valid string - we just need to verify that one is possible.

This led to a fundamentally different approach:

var canBeValid = function(s, locked) {
    if (s.length % 2 !== 0) return false;

    // Check if we have enough potential opening parentheses
    let opens = 0;
    for (let i = 0; i < s.length; i++) {
        if (s[i] === '(' || locked[i] === '0') opens++;
        else opens--;
        if (opens < 0) return false;
    }

    // Check if we have enough potential closing parentheses
    let closes = 0;
    for (let i = s.length - 1; i >= 0; i--) {
        if (s[i] === ')' || locked[i] === '0') closes++;
        else closes--;
        if (closes < 0) return false;
    }

    return true;
};

Learning Impact

This challenge taught me several crucial lessons:

1. Question Interpretation: Always read what the problem is actually asking for. I was asked to check if a valid string was possible, not to create one.

2. Efficiency Through Simplification: The validation approach not only runs faster but uses less memory since it doesn't create any new strings or arrays.

3. Counter vs Creation: Sometimes counting potential outcomes is more powerful than trying to create actual solutions. This is a pattern I've now recognized in many other problems.

4. Problem Reduction: By reducing the problem to "do we have enough flexibility?" instead of "how do we make it valid?", the solution became much clearer.

This experience changed how I approach similar problems. Now, before diving into implementation, I ask myself: "Am I being asked to create a solution, or just verify if one exists?" The difference in approach can be dramatic.

The lesson extends beyond coding challenges. In real-world development, we often over complicate solutions by trying to solve more than what's actually needed. Sometimes, validation is enough - we don't always need to handle the transformation.