leetcode 18. 4Sum

题目

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] ]

解析

package leetcode;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Leet_18 {

public List<List<Integer>> fourSum(int[] nums, int target) {
ArrayList<List<Integer>> res = new ArrayList<>();
int len = nums.length;
if (nums == null || len < 4)
return res;

Arrays.sort(nums);
int max = nums[len - 1];
if (4 * nums[0] > target || 4 * max < target)
return res;

int i, z;
for (i = 0; i < len; i++) {
z = nums[i];
if (i > 0 && z == nums[i - 1])
continue;
if (z + 3 * max < target)   //z is too small
continue;
if (4 * z > target) // z is too large
break;
if (4 * z == target) {
if (i + 3 < len && nums[i + 3] == z)
res.add(Arrays.asList(z, z, z, z));
break;
}

threeSumForFourSum(nums, target - z, i + 1, len - 1, res, z);
}
return res;
}

public void threeSumForFourSum(int[] nums, int target, int low, int high, ArrayList<List<Integer>> fourSumList, int z1) {
if (low + 1 >= high)
return ;

int max = nums[high];
if (3 * max < target || 3 * nums[low] > target)
return ;

int i, z;
for (i = low; i < high - 1; i++) {
z = nums[i];
if (i > low && z == nums[i - 1])
continue;
if (z + 2 * max < target)
continue;
if (3 * z > target)
break;
if (3 * z == target) {
if (i + 1 < high && nums[i + 2] == z)
fourSumList.add(Arrays.asList(z1, z, z, z));
break;
}
twoSumForFourSum(nums, target - z, i + 1, high, fourSumList, z1, z);
}
}

public void twoSumForFourSum(int[] nums, int target, int low, int high, ArrayList<List<Integer>> fourSumList, int z1, int z2) {
if (low >= high)
return ;
if (2 * nums[low] > target || 2 * nums[high] < target)
return ;

int i = low, j = high, sum, x;
while (i < j) {
sum = nums[i] + nums[j];
if (sum == target) {
fourSumList.add(Arrays.asList(z1, z2, nums[i], nums[j]));
x = nums[i];
while (++i < j && x == nums[i]);

x = nums[j];
while (i < --j && x == nums[j]);
}
if (sum < target)
i++;
if (sum > target)
j--;
}
return ;
}
}