15. 3Sum

最新推荐文章于 2022-12-15 10:04:44 发布

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本文探讨了如何寻找一个整数数组中所有唯一且不同的三个数，使得这三个数相加等于零。通过使用排序和双指针技术，提供了一个有效的解决方案。例如，对于数组[-1,0,1,2,-1,-4]，输出结果应为[[-1,0,1],[-1,-1,2]].

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.

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

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

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]) back--;
            }
            
        }

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

    }
    
    return res;
    
}
};