两数之和

两数之和：给定目标值，求数组中两数之和等于目标值的元素下标。

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