LeetCode18-四数之和

LeetCode18-四数之和

双指针+排序(类似三数之和)

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public List<List<Integer>> fourSum(int[] nums, int target) {
List<List<Integer>> res = new ArrayList<>();
if (nums == null || nums.length < 4) return res;
int length = nums.length;
Arrays.sort(nums);
for (int i = 0; i < nums.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 < nums.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 l = j + 1, r = length - 1;
while (l < r) {
int sum = nums[i] + nums[j] + nums[l] + nums[r];
if (sum == target) {
res.add(new ArrayList<>(Arrays.asList(nums[i], nums[j], nums[l], nums[r])));
while (l < r && nums[l] == nums[++l]) ;
while (l < r && nums[r] == nums[--r]) ;
} else if (sum < target) {
l++;
}else {
r--;
}
}
}
}
return res;
}

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