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mediumDSA

What is the two-pointer technique?

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01

Understand the problem

Explain two pointers.

two-pointers
02

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03

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Two pointers uses two indices moving through data (often sorted) to reduce nested loops to O(n) — e.g. finding a pair summing to a target, reversing in place, or removing duplicates. A sliding-window variant maintains a range for substring/subarray problems.

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04

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Pair sum in a sorted array
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def pair_sum(arr, target):
    left, right = 0, len(arr) - 1
    while left < right:
        s = arr[left] + arr[right]
        if s == target:
            return (left, right)
        if s < target:
            left += 1
        else:
            right -= 1
    return None


# --- demo ---
print(pair_sum([2, 7, 8, 11, 15, 18], 18))   # (1, 3)  -> 7 + 11
05

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