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:
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)
The algorithm is similar to 3Sum.
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public
class
Solution { public
ArrayList<ArrayList<Integer>> fourSum(int[] num, int
target) { ArrayList<ArrayList<Integer> > result = new
ArrayList<ArrayList<Integer> >(); int
len = num.length; if(len >=4){ // sort array first Arrays.sort(num); for(int
i = 0; i < len - 3; ++i){ int
j = i + 1; int
sum = target - num[i]; while(j < len - 2){ int
mid = j + 1; int
end = len - 1; while(mid < end){ int
tripleSum = num[j] + num[mid] + num[end]; if(tripleSum == sum){ ArrayList<Integer> oneSolution = new
ArrayList<Integer>(); oneSolution.add(num[i]); oneSolution.add(num[j]); oneSolution.add(num[mid]); oneSolution.add(num[end]); result.add(oneSolution); } else
if(tripleSum < sum){ ++mid; continue; } else{ --end; continue; } ++mid; --end; while(mid < end && num[mid - 1] == num[mid]) ++mid; while(mid < end && num[end] == num[end + 1]) --end; } ++j; while(j < len - 2
&& num[j - 1] == num[j]) ++j; } while(i < len - 3
&& num[i] == num[i + 1]) ++i; } } return
result; }} |
原文：http://www.cnblogs.com/averillzheng/p/3617061.html