Two Pointer Technique on Arrays in DSA C - Execution Trace

Concept Flow - Two Pointer Technique on Arrays

Initialize two pointers: left=0, right=end

↓

Check condition: left < right?

No→Stop

Yes↓

Process elements at left and right

↓

Move pointers: left++ or right-- or both

↩Back to Check condition

Start with two pointers at array ends, check condition, process elements, move pointers inward, repeat until pointers meet.

Execution Sample

DSA C

int left = 0, right = n - 1;
while (left < right) {
  if (arr[left] + arr[right] == target) {
    // found pair
    break;
  } else if (arr[left] + arr[right] < target) {
    left++;
  } else {
    right--;
  }
}

Find two numbers in a sorted array that sum to a target using two pointers moving inward.

Execution Table

Step	Operation	left	right	Sum arr[left]+arr[right]	Action	Visual State
1	Initialize pointers	0	5	arr[0]+arr[5]=1+12=13	13 < 15, move left++	[1, 3, 5, 7, 9, 12] (left=0, right=5)
2	Move left pointer	1	5	arr[1]+arr[5]=3+12=15	Sum == target, found pair	[1, 3, 5, 7, 9, 12] (left=1, right=5)
3	Break loop	1	5	15	Stop searching	[1, 3, 5, 7, 9, 12] (left=1, right=5)

💡 Pointers met condition sum == target, loop stops.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
left	0	1	1	1
right	5	5	5	5
sum	N/A	13	15	15

Key Moments - 3 Insights

Why do we move the left pointer when the sum is less than the target?

Because the array is sorted, increasing left moves to a bigger number, increasing the sum. See execution_table step 1 where sum 13 < 15, so left moves from 0 to 1.

Why do we stop when left pointer meets right pointer?

When left >= right, all pairs have been checked. In execution_table step 3, the loop stops because the pair sum equals target, or pointers crossed.

What if the array is not sorted?

The two pointer technique relies on sorting to decide pointer moves. Without sorting, this logic fails, so the method won't find pairs correctly.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'left' after step 1?

A 1

B 0

C 5

D N/A

💡 Hint

Check the 'left' column in execution_table row for step 1.

At which step does the sum of arr[left] + arr[right] equal the target?

A Step 1

B Step 2

C Step 3

D Never

💡 Hint

Look at the 'Sum arr[left]+arr[right]' and 'Action' columns in execution_table.

If the array was not sorted, what would happen to the pointer movement logic?

A It would still work correctly

B Pointers would move randomly

C The logic to move pointers based on sum comparison would fail

D The pointers would never move

💡 Hint

Refer to key_moments explanation about sorting necessity.

Concept Snapshot

Two Pointer Technique on Arrays:
- Use two pointers at start and end of sorted array
- Check sum of elements at pointers
- If sum < target, move left pointer right
- If sum > target, move right pointer left
- Stop when pointers meet or pair found

Full Transcript

The two pointer technique uses two indexes starting at the ends of a sorted array. We check the sum of the elements at these pointers. If the sum is less than the target, we move the left pointer right to increase the sum. If the sum is greater, we move the right pointer left to decrease the sum. We repeat this until the pointers meet or we find the target sum. This method works only on sorted arrays because the pointer movement depends on the order of elements.