1 Two Sum 05/17

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].

Use hashmap to store the difference between the current number and the target, map it to index of the element. As you loop through the array, when you find the difference is the current value, you get one answer.

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