Why Your Sliding Window "Shrinks Wrong" — A Debugging Guide

You have the logic in your head: "Expand right, shrink left." You write the code. You run the test case.
Output: 0. Or IndexOutOfBounds. Or an infinite loop.

Sliding Window seems simple, but the shrinking logic is where 90% of bugs hide. In this guide, we will debug the most common reasons why your window "shrinks wrong" and give you a checklist to fix it.

TL;DR — Debugging Checklist

• The Loop: Use while (invalid) to shrink, not if.
• The Order: Expand -> Shrink -> Update Result.
• The Update: Only update max/min when the window is valid (usually after the shrink loop).
• The Trap: Negative numbers break the "shrink fixes it" logic.

Common bug scenarios

1. The "If" instead of "While" trap

You check if the window is invalid, remove one element, and continue.

• Bug: The window might still be invalid after removing one element!
• Symptom: Your window size is larger than allowed, or sums are slightly off.
• Fix: Always use while (invalid) { shrink }.

2. Left pointer moves too early (or too late)

• Scenario: You increment left before updating the window state (e.g., removing from sum).
• Symptom: The sum/count excludes an element that is still technically inside the window boundaries [left, right].

3. Window condition mis-checked

• Scenario: You check sum == K inside the shrink loop instead of outside.
• Symptom: You miss valid windows because you shrank past them.

Pattern: “expand while condition valid → shrink when invalid”

The most robust mental model to prevent shrinking bugs is:

1. Expand (add nums[right]).
2. Loop (while condition is broken).
3. Shrink (remove nums[left], increment left).
4. Check/Update Result (now that window is valid).

Visualizing the State:
At the end of the while loop, the window [left, right] is guaranteed to be valid (or empty). This is the safe time to update maxLength.

Debugging checklist

If your code is failing, run through this list:

1. Print Window Boundaries: Inside the loop, print left, right, and the current sum or map.
   • console.log(L:$leftR:${right} Sum:${sum})
2. Replicate with Simple Data: Don't debug with the massive failed test case. Use [1, 1, 10, 1] or "abcba".
3. Check Edge Cases:
   • Does it handle the first element correctly?
   • Does it handle an empty array?
   • Negative Numbers: Remember, if you have negatives, shrinking might not fix the condition. (See When NOT to Use).

Real code example: Incorrect vs Correct

Problem: Find the length of the smallest subarray with sum $\ge 7$.

❌ The Buggy Version

function minSubArrayLen(target, nums) {
    let left = 0, sum = 0, minLen = Infinity;
    for (let right = 0; right < nums.length; right++) {
        sum += nums[right];
        if (sum >= target) { // BUG: Should be 'while'
            minLen = Math.min(minLen, right - left + 1);
            sum -= nums[left];
            left++;
        }
    }
    return minLen === Infinity ? 0 : minLen;
}

Why it fails: If nums = [1, 1, 1, 1, 100] and target = 7. When we hit 100, sum is 104. We shrink once (remove 1). Sum is 103. Loop ends. We report length 4. Correct answer is 1 (just [100]).

✅ The Corrected Version

function minSubArrayLen(target, nums) {
    let left = 0, sum = 0, minLen = Infinity;
    for (let right = 0; right < nums.length; right++) {
        sum += nums[right];
        while (sum >= target) { // FIXED: Keep shrinking until invalid
            minLen = Math.min(minLen, right - left + 1); // Update result while valid
            sum -= nums[left];
            left++;
        }
    }
    return minLen === Infinity ? 0 : minLen;
}

Tooling & visualization tips

• Paper Tracing: Draw the array. Use two fingers. Move them exactly as your code dictates.
• Debugger: Set a conditional breakpoint on right.
• Invariant Check: Ask yourself: "At line X, is sum exactly equal to the sum of nums[left...right]?"

Summary

Shrinking bugs usually come from under-shrinking (using if) or updating state out of order. Stick to the standard pattern: Expand -> While Invalid Shrink -> Update Result.

Next Step:
Even if your logic is right, you might still get the length wrong by one index. Fix that with Sliding Window Off-by-One Errors.