Coding Spirit

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Algorithm Puzzles: 3Sum

Algorithm Puzzles everyday every week sometimes: 3Sum

Puzzle

Puzzle from leetcode:

Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0.

Notice that the solution set must not contain duplicate triplets.

Example 1:

Input: nums = [-1,0,1,2,-1,-4]
Output: [[-1,-1,2],[-1,0,1]]

Example 2:

Input: nums = []
Output: []

Example 3:

Input: nums = [0]
Output: []

Solution

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class Solution {
public:
std::vector<std::vector<int>> threeSum(std::vector<int>& nums) {
if (nums.size() < 3) {
return res;
}

std::sort(nums.begin(), nums.end());

for (int i = 0; i < nums.size() - 2; ++i) {
if (i >= 1 && nums[i] == nums[i - 1]) {
continue;
} else {
twoSum(nums, nums[i], i + 1, nums.size() - 1);
}
}

return res;
}

private:
std::vector<std::vector<int>> res;

void twoSum(const std::vector<int>& nums, const int& target, const int& left,
const int& right) {
if (left >= right) {
return;
}

int sum = nums[left] + nums[right] + target;

if (sum == 0) {
// std::vector<int> mached = ;
res.emplace_back(std::vector<int>{target, nums[left], nums[right]});
}

int i = 0;
if (sum <= 0) {
for (i = 1; i < right - left; ++i) {
if (nums[left] != nums[left + i]) {
break;
}
}
twoSum(nums, target, left + i, right);
} else {
for (i = 1; i < right - left; ++i) {
if (nums[right] != nums[left - i]) {
break;
}
}
twoSum(nums, target, left, right - i);
}
}
};