(Java)LeetCode-18. 4Sum

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

Note:

Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
The solution set must not contain duplicate quadruplets.

    For example, given array S = {1 0 -1 0 -2 2}, and target = 0.    A solution set is:    (-1,  0, 0, 1)    (-2, -1, 1, 2)    (-2,  0, 0, 2)

这道题我用的思路和上一题差不多，三层循环，最后一层循环有两个指针从两边往中间遍历。注意要排除一些情况可以大大的提高速度。注意可排除的情况和前几题不一样哦。还有一种思路是用HashMap来解，我大概看了看感觉好难好麻烦，就还是就这个暴力的解法吧，话说可以击败90%的人

public class Solution {    public List<List<Integer>> fourSum(int[] nums, int target) {        Arrays.sort(nums);List<List<Integer>> result = new ArrayList<List<Integer>>();int length = nums.length;int pre_i = Integer.MAX_VALUE;for(int i = 0; i < length - 3; i++){if(pre_i == nums[i]){continue;}pre_i = nums[i];if(4*nums[i] > target){        break;    }int remain_target = target-nums[i];if(nums[i] + 3*nums[i+1] > target || nums[i] + 3*nums[length-1] < target){continue;}for(int j = i+1; j < length - 2; j++){if(j > i+1 && nums[j-1]==nums[j]){continue;}int m = j + 1;int n = length-1;int sub_target = target-nums[i]-nums[j];while(m < n){if(nums[m] + nums[n] == sub_target){Integer[] temp = {nums[i],nums[j],nums[m],nums[n]};result.add(Arrays.asList(temp));while(++m < n && nums[m] == nums[m-1]);while(m < --n && nums[n] == nums[n+1]);}else if(nums[m] + nums[n] < sub_target){while(++m < n && nums[m] == nums[m-1]);}else{while(m < --n && nums[n] == nums[n+1]);}}}}return result;    }}

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