1.Two sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

Solution: First think binary search. Might not be a good solution since it needs first to be sort O(NlogN), Then search O(nlogn).

HashMap may be a good way.

public int[] twoSum(int[] nums, int target) {
        int[] result=new int[2];
       HashMap<Integer,Integer> map=new HashMap<Integer,Integer>();
        for(int i=0;i<nums.length;i++){
            if(map.get(target-nums[i])!=null){
                result[0]=map.get(target-nums[i]);
                result[1]=i;
                return result;
            }
            map.put(nums[i],i);
        }
        return result;
    }

2.Three Sum

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Solution: Brute force would be O(N^3).

In whatever way we must traverse firstly to ensure one number, then the problem would be two sum.

Hashset:O(N*N)

Sort+Two pointers: O(NlogN+N^n)