题目：

链接: link

方法：

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暴力代码：

public int[] twoSum(int[] nums, int target) {解法一：暴力解法：int n = nums.length;for(int i = 1; i <= n; i++)for(int j = i-1; j >= 0; j--){if(target == nums[i] + nums[j]){return new int[]{i,j};}}return null;    }

优化后代码：

class Solution {//解法二：用哈希表优化：Map<Integer,Integer> hash = new HashMap<>();//<nums[i],i>for(int i = 0; i < nums.length; i++){int find = target - nums[i];if(hash.containsKey(find)){return new int[]{i,hash.get(find)};}hash.put(nums[i],i);}return null;}
}