#yyds干货盘点# LeetCode 热题 HOT 100:四数之和

题目:

给你一个由 n 个整数组成的数组 nums ,和一个目标值 target 。请你找出并返回满足下述全部条件且不重复的四元组 [nums[a], nums[b], nums[c], nums[d]] (若两个四元组元素一一对应,则认为两个四元组重复):

0 <= a, b, c, d < n

a、b、c 和 d 互不相同

nums[a] + nums[b] + nums[c] + nums[d] == target

你可以按 任意顺序 返回答案 。

 

示例 1:

输入:nums = [1,0,-1,0,-2,2], target = 0

输出:[[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]

示例 2:

输入:nums = [2,2,2,2,2], target = 8

输出:[[2,2,2,2]]

class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
List<List<Integer>> quadruplets = new ArrayList<List<Integer>>();
if (nums == null || nums.length < 4) {
return quadruplets;
}
Arrays.sort(nums);
int length = nums.length;
for (int i = 0; i < length - 3; i++) {
if (i > 0 && nums[i] == nums[i - 1]) {
continue;
}
if ((long) nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3] > target) {
break;
}
if ((long) nums[i] + nums[length - 3] + nums[length - 2] + nums[length - 1] < target) {
continue;
}
for (int j = i + 1; j < length - 2; j++) {
if (j > i + 1 && nums[j] == nums[j - 1]) {
continue;
}
if ((long) nums[i] + nums[j] + nums[j + 1] + nums[j + 2] > target) {
break;
}
if ((long) nums[i] + nums[j] + nums[length - 2] + nums[length - 1] < target) {
continue;
}
int left = j + 1, right = length - 1;
while (left < right) {
long sum = (long) nums[i] + nums[j] + nums[left] + nums[right];
if (sum == target) {
quadruplets.add(Arrays.asList(nums[i], nums[j], nums[left], nums[right]));
while (left < right && nums[left] == nums[left + 1]) {
left++;
}
left++;
while (left < right && nums[right] == nums[right - 1]) {
right--;
}
right--;
} else if (sum < target) {
left++;
} else {
right--;
}
}
}
}
return quadruplets;
}
}
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