Basic two-sum
Input
nums = [2,7,11,15], target = 9
Output
[0, 1]
nums[0] + nums[1] = 9.
Full lesson preview
Find two indices with target sum
Find two distinct indices whose elements sum to a target using a hash map for O(n) time.
Python practice15 minArrays – Fundamentals & PatternsIntermediateLast updated March 25, 2026
Problem statement
Given an array of integers nums and an integer target, return a list [i, j] of two distinct 0-based indices such that nums[i] + nums[j] == target. If multiple pairs exist, returning any valid pair is acceptable. If no such pair exists, return [-1, -1]. Use an algorithm with O(n) time and O(n) extra space.
Task
Return indices of two numbers that add up to the target. If none exist, return [-1, -1].
Examples
Input format
Function call: two_sum_indices(nums, target) where nums is a list of integers and target is an integer.
Output format
A list of two integers [i, j] representing indices. If no pair exists, return [-1, -1].
Constraints
2 <= len(nums) <= 10^5. Elements may be negative. Do not use the same element twice (indices must be distinct). Time: O(n), Space: O(n).
Samples
Sample 1
Input
two_sum_indices([3,2,4], 6)
Output
[1, 2]
nums[1] + nums[2] = 6.