/*
1. 反向索引每个数的下标，查找target-numgei[i]是否存在
2. 必须是2个数字所以需要特判下是否是自身组成的
3. 时间O（N），空间O（N）
*/


class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> mapping = new HashMap<>();
        for (int i = 0 ; i < nums.length ; i ++){
            mapping.put(nums[i], i);
        }

        for (int i = 0 ; i < nums.length; i ++){
            int diff = target - nums[i];
            if (mapping.containsKey(diff)){
                int index = mapping.get(diff);
                if (i == index) continue;
                int[] res = {i, index};
                return res;
            }
        }
        return null;
    }
}