算法分析与设计课程——LeetCode刷题之3Sum

题目：

Given an array S of n integers, are there elements a, b, c in S such that a + b + c =
0? Find all unique triplets in the array which gives the sum of zero.
Note: The solution set must not contain duplicate triplets.

For example, given array S = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
[-1, 0, 1],
[-1, -1, 2]
]

answer:

class Solution {
public:
vector<vector<int> > threeSum(vector<int> &num) {

vector<vector<int> > res;

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

for (int i = 0; i < num.size(); i++) {

int target = -num[i];
int front = i + 1;
int back = num.size() - 1;

while (front < back) {

int sum = num[front] + num[back];

// Finding answer which start from number num[i]
if (sum < target)
front++;

else if (sum > target)
back--;

else {
vector<int> triplet(3, 0);
triplet[0] = num[i];
triplet[1] = num[front];
triplet[2] = num[back];
res.push_back(triplet);

// Processing duplicates of Number 2
// Rolling the front pointer to the next different number forwards
while (front < back && num[front] == triplet[1]) front++;

// Processing duplicates of Number 3
// Rolling the back pointer to the next different number backwards
while (front < back && num[back] == triplet[2]) rear--;
}

}

// Processing duplicates of Number 1
while (i + 1 < num.size() && num[i + 1] == num[i])
i++;

}

return res;

}
};