边往set里放，边比较

class Solution {
    public int[] findNumbersWithSum(int[] nums, int target) {
        if(nums==null || nums.length<=1) return new int[2];
        Set<Integer> set=new HashSet();
        for(int num:nums){
            int t=target-num;
            if(set.contains(t)){
                return new int[]{num,t};
            }else{
                set.add(num);
            }
        }
        return null;
    }
}