LeetCode18. 4Sum

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:

• Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
• 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)

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实现：

class Solution {public:    void twoSum(vector<int>& nums, int target, int start, int end, vector<vector<int>>& vvi, int z1, int z2){if (start >= end) {return;}if (2 * nums[start] > target || 2 * nums[end] < target) {return;}int i = start;int j = end;int sum, x;while (i < j) {sum = nums[j] + nums[i];if (sum == target) {vvi.push_back(vector<int> {z1, z2, nums[i], nums[j]});x = nums[i];while (++i < j && nums[i] == x) {;}x = nums[j];while (--j > i && nums[j] == x) {;}}else if (sum < target) {i++;}else {j--;}}}void threeSum(vector<int>& nums, int target, int start, int end, vector<vector<int>>& vvi, int z1){if (start + 1 >= end) return;int max = nums[end];if (3 * nums[start] > target || 3 * max < target) {return;}int i, z;for (i = start; i < end - 1; i++) {z = nums[i];if (i > start && nums[i - 1] == z) {continue;}if (z + 2 * max < target) {continue;}if (3 * z > target) {break;}if (3 * z == target) {if (i + 1 < end && nums[i + 2] == z) {vvi.push_back(vector<int>{z1, z, z, z});}break;}twoSum(nums, target - z, i + 1, nums.size() - 1, vvi, z1, z);}}vector<vector<int>> fourSum(vector<int>& nums, int target) {vector<vector<int>> vvi;if (nums.size() < 4) {return vvi;}std::sort(nums.begin(), nums.end());int max = nums[nums.size() - 1];if (4 * nums[0] > target || 4 * max < target)return vvi;int i, z;for (i = 0; i < nums.size(); i++) {z = nums[i];if (i > 0 && z == nums[i - 1]) {continue;}if (z + 3 * max < target) {continue;}if (z * 4 > target) {break;}if (4 * z == target) {if (i + 3 < nums.size() && nums[i + 3] == z) {vvi.push_back(vector<int>{z, z, z, z});}break;}threeSum(nums, target - z, i + 1, nums.size() - 1, vvi, z);}return vvi;}};