跟三数之和的解法差不多,只是多了一重for循环,现在时间复杂度为O(n^3)。
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class LeetCode18 {
public static void main(String[] args) {
List<List<Integer>> res = new ArrayList<>();
res = fourSum(new int[]{1,0,-1,0,-2,2}, 0);
System.out.println("success");
}
public static List<List<Integer>> fourSum(int[] nums, int target) {
List<List<Integer>> result = new ArrayList<>();
if (nums != null && nums.length > 3) {
Arrays.sort(nums);
for (int i = 0; i < nums.length - 3; ) {
int stepRes = target - nums[i];
for (int w = i + 1; w < nums.length - 2; ){
int j = w + 1;
int k = nums.length - 1;
while (j < k) {
if (nums[j] + nums[k] == -nums[w] + stepRes) {
List<Integer> list = new ArrayList<>(3);
list.add(nums[i]);
list.add(nums[w]);
list.add(nums[j]);
list.add(nums[k]);
result.add(list);
k--;
j++;
while (j < k && nums[j] == nums[j - 1]) {
j++;
}
while (j < k && nums[k] == nums[k + 1]) {
k--;
}
} else if (nums[j] + nums[k] > -nums[w] + stepRes) {
k--;
while (j < k && nums[k] == nums[k + 1]) {
k--;
}
} else {
j++;
while (j < k && nums[j] == nums[j - 1]) {
j++;
}
}
}
w++;
while (w < nums.length - 2 && nums[w] == nums[w - 1]) {
w++;
}
}
i++;
while (i < nums.length - 3 && nums[i] == nums[i - 1]) {
i++;
}
}
}
return result;
}
}