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Friday, January 25, 2013

[LeetCode] 3 Sum, Solution


Given an array S of n integers, are there elements abc in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.
Note:
  • Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
  • 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)
» Solve this problem

[Thoughts]
Two-pointer scan.

[Code]
1:    vector<vector<int> > threeSum(vector<int> &num) {  
2:      // Start typing your C/C++ solution below  
3:      // DO NOT write int main() function  
4:      std::sort(num.begin(), num.end());  
5:      vector<vector<int> > result;  
6:      int len = num.size();  
7:      for(int i =0; i< len; i++)  
8:      {  
9:        int target = 0-num[i];  
10:        int start = i+1, end =len-1;  
11:        while(start<end)  
12:        {  
13:          if(num[start] + num[end] == target)  
14:          {  
15:            vector<int> solution;  
16:            solution.push_back(num[i]);  
17:            solution.push_back(num[start]);  
18:            solution.push_back(num[end]);  
19:            result.push_back(solution);  
20:            start++; end--;  
21:            while(start<end && num[start] == num[start-1]) start++;  
22:            while(start<end && num[end] == num[end+1]) end--;  
23:          }  
24:          else if(num[start] + num[end] < target)  
25:          {  
26:            start++;  
27:          }  
28:          else  
29:          {  
30:            end--;  
31:          }  
32:        }  
33:        if(i<len-1)  
34:        {  
35:          while(num[i] == num[i+1]) i++;  
36:        }  
37:      }  
38:      return result;  
39:    }  

[Some tricks]
1. Line 21 and Line 22.
    filter the duplicate during two-pointer scan. For example [-2, 0, 0, 2,2], the expected output should be [-2,0,2]. If no filter here, the output will be duplicate as [-2,0,2] and [-2,0,2]
2. Line 35
   filter the duplicate for outside iteration. For example [-2, -2, -2, 0,2].
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