Two pointer is really an easy and effective technique which is typically used for searching pairs in a sorted arrays.
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 input array is sorted. We start sum of extreme values (smallest and largest) and conditionally move both pointers. We move left pointer i when sum of A[i] and A[j[ is less than X. We do not miss any pair because 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
- C++ Program to compare two string using pointers
- Program to reverse an array using pointers
- Repeated Character Whose First Appearance is Leftmost
- Queries for bitwise AND in the index range [L, R] of the given array
- Find number from its divisors
- Queries for bitwise OR in the index range [L, R] of the given array
- Minimum steps required to reach the end of a matrix | Set 2
- Find the index which is the last to be reduced to zero after performing a given operation
- Queries to find the maximum Xor value between X and the nodes of a given level of a perfect binary tree
- Minimize the number of steps required to reach the end of the array | Set 2
- Count index pairs which satisfy the given condition
- Maximum sum such that no two elements are adjacent | Set 2
- Minimize the number of steps required to reach the end of the array
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