up:: [[Algorithms MOC]]
# Two Pointer or Iterator Algorithm
In this algorithm, you keep track of two pointers to items in an array, and iterate one or the other as needed. This works with sorted lists.
## When To Use
This algorithm is used when you have a sorted list and you need to operate on pairs, triplets, or more. This is a good first choice when you need to find two numbers in an array that satisy some condition.
## Examples
```python
def isPairSum(A, N, X):
# represents first pointer
i = 0
# represents second pointer
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 towards
# higher values by doing i += 1
elif(A[i] + A[j] < X):
i += 1
# If sum of elements at current
# pointers is more, we move towards
# lower values by doing j -= 1
else:
j -= 1
return 0
# array declaration
arr = [2, 3, 5, 8, 9, 10, 11]
# value to search
val = 17
print(isPairSum(arr, len(arr), val))
```
### Problems Featuring this Solution
- Squaring a sorted array (easy)
- Triplets that sum to zero (medium)
- Comparing strings that contain backspaces (medium)
## Sources
[[14 Patterns to Ace Any Coding Interview Question#2 Two Pointers or Iterators 336855299|14 Patterns to Ace Coding Interview #2: 2 Pointer]]
[[727. Remove Duplicates From Sorted Array#^ymwuzy]]