Two Pointer Technique in Java: The Trick That Replaces Nested Loops

#ai #database #coding #programming

Many array problems look like they need nested loops.

That means O(n²) time.

But with one simple idea — Two Pointers —
you can often reduce it to O(n).

What is Two Pointer Technique?

Instead of using one loop,
you use two indices (pointers):

From opposite ends
Or moving in the same direction

Helps reduce unnecessary work

Example 1: Two Sum (Sorted Array)

public static int[] twoSumSorted(int[] arr, int target) {
int left = 0, right = arr.length - 1;

while (left < right) {
    int sum = arr[left] + arr[right];

    if (sum == target) return new int[]{left, right};
    else if (sum < target) left++;
    else right--;
}

return new int[]{-1, -1};

}

Idea:

If sum is small → move left
If sum is large → move right

Example 2: Remove Duplicates

public static int removeDuplicates(int[] arr) {
if (arr.length == 0) return 0;

int writeIndex = 1;

for (int readIndex = 1; readIndex < arr.length; readIndex++) {
    if (arr[readIndex] != arr[readIndex - 1]) {
        arr[writeIndex++] = arr[readIndex];
    }
}

return writeIndex;

}
Idea:

One pointer reads
One pointer writes

Example 3: Container With Most Water

public static int maxArea(int[] height) {
int left = 0, right = height.length - 1, max = 0;

while (left < right) {
    max = Math.max(max,
        (right - left) * Math.min(height[left], height[right])
    );

    if (height[left] < height[right]) left++;
    else right--;
}

return max;

}

Idea: