Two Pointer Technique

When to use: Optimal way to solve problems related to arrays, strings and linked list in O(N) time.

Arrays/Strings: Two pointers, each starting from the beginning and the end until they both meet.
Example Problem: Two Sum II - Input Array Is Sorted

class Solution {
    public int[] twoSum(int[] numbers, int target) {
        // use two pointer technique because the input is sorted.
        int start = 0; 
        int end = numbers.length - 1;
        int [] result = new int [2];

        while (start < end){
            int sum = numbers[start] + numbers[end];

            if (sum == target){
                result[0] = start + 1;
                result[1] = end + 1;
                break;
            }

            if (sum < target){
                start++;
            } else {
                end--;
            }
        }
        return result;
    }
}

Linked List: One pointer moving at a slow pace, while the other pointer moves at twice the speed.
Example Problem: Linked List Cycle

public class Solution {
    public boolean hasCycle(ListNode head) {

        if(head == null){
            return false;
        }

        ListNode slow = head; 
        ListNode fast = head;

        while(fast != null && fast.next != null){

            slow = slow.next;
            fast = fast.next.next;

            if(slow == fast){
                return true;
            }
        }
        return false;
    }
}