1. Two sum

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