Leetcode 1 2sum:

HashMap to record Key value pair, get(target-nums[i]) or put the value

Knowledge: Map to get and put and containsKey

                Array for loop

space: o(1)

time: o(n) 

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