1. Two Sum

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

HashMap

Complexity

O(N) O(1)

Code

class Solution {
    public int[] twoSum(int[] nums, int target) {
        if(nums == null || nums.length == 0)
            return new int[0];
        int[] res = new int[2];
        Map<Integer, Integer> map = new HashMap<>();
        for(int i = 0; i < nums.length; i++){
            if(map.containsKey(target - nums[i])){
                res[0] = map.get(target - nums[i]);
                res[1] = i;
            }else{
                map.put(nums[i], i);
            }
        }
        return res;
    }
}