[Leetcode] 4Sum

题目

Given anarray S of n integers, are thereelements a, b, c, and d in S suchthat a + b + c + d = target? Find allunique 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.

 

    Asolution set is:

   (-1,  0, 0, 1)

   (-2, -1, 1, 2)

   (-2,  0, 0, 2)

 

思路

有了2Sum，3Sum的基础，这道题不难。前面在解决3Sum的时候转化为2Sum来求解，这里同样可以把4Sum转化为3Sum来求解，需要注意的是去除重复结果。

 

代码

public class Solution {    List<List<Integer>> result =new ArrayList<List<Integer>>();    public List<List<Integer>>fourSum(int[] nums, int target) {        if (nums == null)            return result;        int len = nums.length;        if (len < 4)            return result;        Arrays.sort(nums);               for (int i = 0; i < len - 3; i++) {            if (i > 0 && nums[i] ==nums[i-1])                continue;            threeSum(nums, i + 1, len - 1,target, nums[i]);        }        return result;    }       public void threeSum(int[] nums, int l, intr, int target, int num1) {        for (int i = l; i <= r - 2; i++) {            if (i > l && nums[i] ==nums[i-1])                continue;            twoSum(nums, i + 1, r, target,num1, nums[i]);        }    }       public void twoSum(int[] nums, int l, intr, int target, int num1, int num2){        while (l < r) {            if (num1 + num2 + nums[l] + nums[r]== target) {                List<Integer> list = newArrayList<Integer>();                list.add(num1);                list.add(num2);                list.add(nums[l]);                list.add(nums[r]);                result.add(list);                while (l < r && nums[l] ==nums[l+1])                    l++;                while (l < r &&nums[r] == nums[r-1])                    r--;                l++;                r--;            }else if (num1 + num2 + nums[l] +nums[r] < target) {                l++;            }else {                r--;            }        }    }}