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Solution.java
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Solution.java
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package _0018_4Sum;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
/**
* 使用双索引技术解决
* 时间复杂度:O(n*logn + n^3)
* 空间复杂度:O(1)
*/
public class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
int n = nums.length;
List<List<Integer>> res = new ArrayList<>();
if (n <= 3) return res;
// 数组排序,默认升序排列
Arrays.sort(nums);
for (int i = 0; i <= n - 4; i = next_num_index(nums, i))
for (int j = i + 1; j <= n - 3; j = next_num_index(nums, j)) {
// 待查找的目标t
int t = target - nums[i] - nums[j];
if (nums[j + 1] + nums[j + 2] > t || nums[n - 1] + nums[n - 2] < t) continue;
int p1 = j + 1, p2 = n - 1;
while (p1 < p2) {
if (nums[p1] + nums[p2] == t) {
res.add(generateList(nums[i], nums[j], nums[p1], nums[p2]));
p1 = next_num_index(nums, p1);
p2 = pre_num_index(nums, p2);
} else if (nums[p1] + nums[p2] < t)
p1 = next_num_index(nums, p1);
else
p2 = pre_num_index(nums, p2);
}
}
return res;
}
private List<Integer> generateList(int a, int b, int c, int d) {
List<Integer> item = new ArrayList<>();
item.add(a);
item.add(b);
item.add(c);
item.add(d);
return item;
}
// 返回nums[index+1...n)中第一个不等于nums[index]的数据的下标
private int next_num_index(int[] nums, int index) {
for (int i = index + 1; i < nums.length; i++)
if (nums[i] != nums[index])
return i;
return nums.length;
}
// 返回nums[0...index-1]中第一个不等于nums[index]的数据的下标
private int pre_num_index(int[] nums, int index) {
for (int i = index - 1; i >= 0; i--)
if (nums[i] != nums[index])
return i;
return -1;
}
public static void main(String[] args) {
int[] nums = {1, 0, -1, 0, -2, 2};
System.out.println((new Solution()).fourSum(nums, 0));
}
}