Code Quiz

Two-Pointer Pair Sum Bug

Spot the pointer-movement mistake in a classic two-pointer sorted-array two-sum solution.

Codejavascript

// arr is sorted ascending; return indices of two values summing to target
function twoSumSorted(arr, target) {
  let left = 0, right = arr.length - 1;
  while (left < right) {
    const sum = arr[left] + arr[right];
    if (sum === target) return [left, right];
    else if (sum < target) right--;
    else left++;
  }
  return [-1, -1];
}

console.log(twoSumSorted([1, 2, 4, 7, 11], 9)); // expected [1, 3]

Why does this two-pointer function fail to find valid pairs?

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