leetcode — 4sum

import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

/**
* Source : https://oj.leetcode.com/problems/4sum/ *
* Created by lverpeng on 2017/7/10.
*
* 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)
*/
public class FourSum {

/**
*
* 分解为多个3sum问题，求出3sum问题普通解法
* 3sum的解法就是分为多个2sum问题，2sum问题就可以使用原来的方法进行解答
*/
public Set<Integer[]>  threeSum (int[] num, int total) {
Set<Integer[]> set = new HashSet<Integer[]>();
//        Arrays.sort(num);
for (int i= 0; i < num.length; i++) {
int left = i + 1;
int right = num.length - 1;
int sum2 = total - num[i];
while (left < right) {
if (num[left] + num[right] == sum2) {
Integer[] answer = {num[i], num[left], num[right]};
set.add(answer);
left++;
right--;
while (left < right && num[left] + num[right] == sum2) {
Integer[] ans = {num[i], num[left], num[right]};
set.add(ans);
left++;
right--;
}

} else if (num[left] + num[right] < sum2) {
left++;
while (left < right && num[left] + num[right] < sum2) {
left++;
}
} else {
right--;
while (left < right && num[left] + num[right] > sum2) {
right--;
}
}
}
}
return set;
}

/**
* 对数组排序，分解为3sum处理
*
* @param num
* @param total
* @return
*/
public Set<Integer[]> fourSum (int[] num, int total) {
Set<Integer[]> set = new HashSet<Integer[]>();
Arrays.sort(num);
int[] subArr = new int[num.length - 1];
for (int i = 0; i < num.length; i++) {
int a = num[i];
System.arraycopy(num, i + 1, subArr, 0, num.length  - i - 1);
Set<Integer[]> threeSumResult = threeSum(subArr, total - a);
for (Integer[] intArr : threeSumResult) {
Integer[] arr = new Integer[4];
arr[0] = a;
System.arraycopy(intArr, 0, arr, 1, 3);
set.add(arr);
}
}
return set;
}

public void printList (Set<Integer[]> set) {
for (Integer[] arr : set) {
System.out.println(Arrays.toString(arr));
}
}

public static void main(String[] args) {
FourSum fourSum = new FourSum();
int[] arr = {1, 0, -1, 0, -2, 2};
Set<Integer[]> set  = fourSum.fourSum(arr, 0);
fourSum.printList(set);
}

}