标签：arrays null 注意 解决 ica log color not ret
LeetCode第18题--4Sum
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: 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] ]
1 //解题思路：将之前解决的3Sum问题再加一层循环，注意两层for循环中的条件 2 public class Solution { 3 public List<List<Integer>> fourSum(int[] nums, int target) { 4 List<List<Integer>> result = new LinkedList<>(); 5 if(nums == null || nums.length < 4) { 6 return result; 7 } 8 Arrays.sort(nums);//注意一般情况都是要先进行排序的 9 for(int first = 0; first < nums.length - 3; first++) { 10 if(first > 0 && nums[first] == nums[first - 1]) { 11 continue; 12 } 13 for(int second = first + 1; second < nums.length - 2; second++) { 14 if(second > first + 1 && nums[second] == nums[second - 1]) {//这里要注意的是 second > first + 1 15 continue; 16 } 17 int left = second + 1; int right = nums.length - 1; int target1 = target - (nums[first] + nums[second]); 18 while(left < right) { 19 int sum = nums[left] + nums[right]; 20 if(sum == target1) { 21 result.add(Arrays.asList(nums[first], nums[second], nums[left], nums[right])); 22 while(left < right && nums[left] == nums[left + 1]) left++; 23 while(left < right && nums[right] == nums[right - 1]) right--; 24 left++; 25 right--; 26 } else if(sum < target1) { 27 left++; 28 } else { 29 right--; 30 } 31 } 32 } 33 } 34 return result; 35 } 36 }
标签：arrays null 注意 解决 ica log color not ret
原文地址：http://www.cnblogs.com/muziyueyueniao/p/6706644.html
