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:

  • 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)
在3Sum的基础上稍微修改即可，O(n ^ 3)。
 1 public class Solution {
 2     public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
 3         // Note: The Solution object is instantiated only once and is reused by each test case.
 4          Arrays.sort(num);
 5         ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
 6         if(num == null || num.length < 4) return result;
 7         int len = num.length;
 8         for(int i = 0; i <= num.length - 4; i ++){
 9             if(i==0 || num[i] > num[i-1]){
10                 for(int a = num.length - 1; a > i + 2; a --){
11                     if(a == num.length - 1 || num[a] < num[a + 1]){
12                         int j = i + 1;
13                         int h = a - 1;
14                         while(j < h){
15                             int sum = num[j] + num[h] + num[i] + num[a];
16                             if(sum == target){
17                                 ArrayList<Integer> row = new ArrayList<Integer>();
18                                 row.add(num[i]);
19                                 row.add(num[j]);
20                                 row.add(num[h]);
21                                 row.add(num[a]);
22                                 result.add(row);
23                                 h--;
24                                 j++;
25                                 while(h>j && num[h]==num[h+1]) h--; 
26         
27                                 while(j<h && num[j]==num[j-1]) j++;
28                             }else if(sum < target){
29                                     j ++;
30                             }else if(sum > target){
31                                     h --;
32                             }
33                         }
34                     }
35                 }
36             }
37         }
38         return result;
39     }
40 }

 

posted on 2013-10-17 11:58  Step-BY-Step  阅读(164)  评论(0)  编辑  收藏  举报

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