Question
Given an array of integers, return indices of the two numbers such that they add up to a specific target.
You may assume that each input would have exactly one solution, and you may not use the same element twice.
Example:
Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].
Solution
The idea is to use a hash map map to store target - nums[i] when iterating nums. Once a num[j] is found as an existing key in map, we know nums[j] + map[nums[j]] == target.
Code
class Solution {
public int[] twoSum(int[] nums, int target) {
Map<Integer, Integer> map = new HashMap();
for(int i=0; i<nums.length; i++) {
if(map.containsKey(nums[i]))
return new int[] {map.get(nums[i]), i};
map.put(target-nums[i], i);
}
return null;
}
}
Performance
The solution iterates each element in nums once, so the time complexity is O(n). The hashmap will have at most n entries, so the space complexity is O(n).