3Sum

来源：Leetcode

原帖：http://oj.leetcode.com/problems/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:
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)

代码：

 class Solution {  
 public:  
     vector<vector<int> > threeSum(vector<int> &num) {  
         vector<vector<int>> res;  
         sort(num.begin(), num.end());  
         int N = num.size();  
         for (int i = 0; i < N - 2 && num[i] <= 0; ++i) {  
             if (i != 0 && num[i] == num[i - 1]) continue;  
             int twosum = 0 - num[i]; // any value  
             int l = i + 1, r = N - 1;  
             while (l < r) {  
                 if (l != i + 1 && num[l] == num[l - 1]) { // avoid duplicates  
                     l++; continue;   
                 }   
                 if (r != N - 1 && num[r] == num[r + 1]) { // avoid duplicates  
                     r--; continue;   
                 }   
                 int sum = num[l] + num[r];  
                 if (sum < twosum) {  
                     l++;            
                 } else if (sum > twosum) {  
                     r--;            
                 } else {  
                     vector<int> three = {num[i], num[l], num[r]}; // c++11 new feature initialization list  
                     res.push_back(three);  
                     l++; r--; // do not forget update  
                 }  
             }  
         }  
         return res;  
     }  
 };