Two pointers is really an easy and effective technique which is typically used for searching pairs in a sorted array.
Given a sorted array A (sorted in ascending order), having N integers, find if there exists any pair of elements (A[i], A[j]) such that their sum is equal to X.
Let’s see the naive solution.
The time complexity of this solution is O(n2).
Now let’s see how the two-pointer technique works. We take two pointers, one representing the first element and other representing the last element of the array, and then we add the values kept at both the pointers. If their sum is smaller than X then we shift the left pointer to right or if their sum is greater than X then we shift the right pointer to left, in order to get closer to the sum. We keep moving the pointers until we get the sum as X.
The above solution works in O(n)
How does this work?
The algorithm basically uses the fact that the input array is sorted. We start the sum of extreme values (smallest and largest) and conditionally move both pointers. We move left pointer i when the sum of A[i] and A[j] is less than X. We do not miss any pair because the sum is already smaller than X. Same logic applies for right pointer j.
More problems based on two pointer technique.
- Find the closest pair from two sorted arrays
- Find the pair in array whose sum is closest to x
- Find all triplets with zero sum
- Find a triplet that sum to a given value
- Find a triplet such that sum of two equals to third element
- Find four elements that sum to a given value
- Window Sliding Technique
- Program to reverse an array using pointers
- C++ Program to compare two string using pointers
- Find the number of p-sided squares in a grid with K blacks painted
- Queries for number of distinct elements in a subarray | Set 2
- Length of longest subarray in which elements greater than K are more than elements not greater than K
- Maximum XOR with given value in the path from root to given node in the tree
- Count number of pairs in array having sum divisible by K | SET 2
- Minimum sum of array elements based on given Criteria
- 21 Matchsticks Problem
- Find the smallest positive number which can not be represented by given digits
- Sum of digits with even number of 1's in their binary representation
- Number of possible permutations when absolute difference between number of elements to the right and left are given
- Find all unique pairs of maximum and second maximum elements over all sub-arrays in O(NlogN)
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