Two Sum — Find indices of two numbers that add to target

Question

Accepted Answer

**Approach:** One-pass hash map — for each `num`, check if `target - num` was seen. O(n) time, O(n) space. Handles negatives, duplicates.

**Key steps:** map stores `value -> index`. On iteration, compute complement and test `has`.

```javascript
function twoSum(nums, target) {
  const map = new Map();
  for (let i = 0; i < nums.length; i++) {
    const need = target - nums[i];
    if (map.has(need)) return [map.get(need), i];
    map.set(nums[i], i);
  }
  return [];
}

// Runnable demo
console.log(twoSum([2,7,11,15], 9)); // [0,1]
console.log(twoSum([3,2,4], 6)); // [1,2]
console.log(twoSum([3,3], 6)); // [0,1]
console.log(twoSum([-1, -2, -3, -4], -6)); // [1,3]
```
Edge: return `[]` if not found. Variant: return values not indices — same map but store complement check first.

Two Sum — Find indices of two numbers that add to target

Understand the problem

Attempt it yourself

Study the solution

Solution ready — 2 min read

Join the discussion

Discussion (0)

KEEP THE
STREAK ALIVE.

Understand the problem

Attempt it yourself

Study the solution

Solution ready — 2 min read

Join the discussion

Discussion (0)

KEEP THESTREAK ALIVE.

KEEP THE
STREAK ALIVE.