1. Two Sum

第一次代码：37ms

public class Solution {    public int[] twoSum(int[] nums, int target) {        for(int i = 0; i < nums.length; i++){            for(int j = i + 1; j < nums.length; j++){                if(nums[i]+nums[j] == target)                    return new int[]{i, j};            }        }        return null;    }}

思想：到目前为止是否存在------------比如 i = 10，表示，到时为止，是否能得到target

第二次代码：7ms

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