Sliding Window on Strings Explained Simply in Java

Many string problems look complicated because they involve:

• substrings
• repeated characters
• dynamic conditions

Beginners often use nested loops…

But sliding window reduces many of these problems from:

• O(n²) to:
• O(n)

1. Longest Substring Without Repeating Characters
Problem

Find the length of the longest substring with unique characters.

Efficient Sliding Window Solution

public static int lengthOfLongestSubstring(String s) {

Set<Character> set = new HashSet<>();

int left = 0, maxLen = 0;

for (int right = 0; right < s.length(); right++) {

    while (set.contains(s.charAt(right))) {

        set.remove(s.charAt(left));

        left++;
    }

    set.add(s.charAt(right));

    maxLen = Math.max(maxLen,
            right - left + 1);
}

return maxLen;

}

Core Idea

Maintain a window:

• expand right pointer
• shrink left when duplicate appears

This keeps the substring valid.

*2. Minimum Window Substring
*Problem

Find the smallest substring containing all characters of a pattern.

Example:

Text = "ADOBECODEBANC"
Pattern = "ABC"

Answer:

"BANC"

Key Idea

Use:

• character frequency counts
• dynamic window resizing

Expand until valid,
then shrink to minimize window.

3. Longest Repeating Character Replacement
Problem

Given:

• a string
• k replacements

Find the longest substring that can become all same characters.

Smart Observation

Track:

• most frequent character inside window.

If:

windowSize - maxFreq > k

then shrink window.

Sliding window works because:

• we reuse previous computations instead of recalculating substrings.

That’s the optimization.

• Sliding window avoids nested loops
• Expand → valid state
• Use two pointers
• Track characters using set/map
• Expand right pointer
• Shrink left when condition breaks
• Complexity → O(n)

Sliding window is one of the most important interview patterns.

Once you master it,

• many “hard” string problems become manageable

Explore More: https://www.quipoin.com/tutorial/data-structure-with-java/sliding-window-on-strings