Pattern Blueprint: The Two Pointers Master Template

1. Opposite Ends (The Squeeze)

Use this for sorted arrays where you need to find a pair (e.g., Two Sum II, 3Sum, Reverse String).

public void oppositeEnds(int[] nums) {
    int left = 0, right = nums.length - 1;
    
    while (left < right) {
        int sum = nums[left] + nums[right];
        if (sum == target) {
            // Found result!
            return;
        } else if (sum < target) {
            left++; // Need a larger value
        } else {
            right--; // Need a smaller value
        }
    }
}

2. Fast & Slow (The Tortoise and Hare)

Use this for Linked Lists or cyclic structures to find middle, detect cycles, or find k-th elements.

public void tortoiseAndHare(ListNode head) {
    ListNode slow = head;
    ListNode fast = head;
    
    while (fast != null && fast.next != null) {
        slow = slow.next;        // Move 1 step
        fast = fast.next.next;   // Move 2 steps
        
        if (slow == fast) {
            // Cycle detected!
        }
    }
}

3. Multiple Inputs (The Parallel Scan)

Use this to merge or compare two different sorted arrays (e.g., Merge Sorted Array, Intersection of Lists).

public void parallelScan(int[] arr1, int[] arr2) {
    int i = 0, j = 0;
    
    while (i < arr1.length && j < arr2.length) {
        if (arr1[i] < arr2[j]) {
            i++;
        } else if (arr1[i] > arr2[j]) {
            j++;
        } else {
            // Match found!
            i++; j++;
        }
    }
}

4. Interview Discussion: The "Aha!" Moment

Whenever you see a problem with Sorted Inputs or a need to reduce an $O(N^2)$ nested loop to $O(N)$, Two Pointers is your first line of defense. It leverages the order of the data to eliminate redundant comparisons.