Two Pointers Technique
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.

// Naive solution to find if there is a
// pair in A[0..N1] with given sum.
bool
isPairSum(A[], N, X)
{
for
(i = 0; i < N; i++) {
for
(j = 0; j < N; j++) {
if
(A[i] + A[j] == X)
return
true
;
// pair exists
if
(A[i] + A[j] > X)
break
;
// as the array is sorted
}
}
// No pair found with given sum.
return
false
;
}
The time complexity of this solution is O(n^{2}).
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.

// Two pointer technique based solution to find
// if there is a pair in A[0..N1] with given sum.
bool
isPairSum(A[], N, X)
{
// represents first pointer
int
i = 0;
// represents second pointer
int
j = N  1;
while
(i < j) {
// If we find a pair
if
(A[i] + A[j] == X)
return
true
;
// If sum of elements at current
// pointers is less, we move toward
// higher values by doing i++
else
if
(A[i] + A[j] < X)
i++;
// If sum of elements at current
// pointers is more, we move toward
// higher values by doing i++
else
j;
}
return
false
;
}
Illustration :
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.
Disclaimer: This does not belong to TechCodeBit, its an article taken from the below
source and credits.
source and credits:http://www.geeksforgeeks.org/twopointerstechnique/
We have built the accelerating growthoriented website for budding engineers and aspiring job holders of technology companies such as Google, Facebook, and Amazon
If you would like to study our free courses you can join us at
http://www.techcodebit.com. #techcodebit #google #microsoft #facebook #interview portal #jobplacements
#technicalguide