Leetcode#15 3Sum

原题地址

 

跟2sum、3sum、4sum、3sum closest一系列，参见这篇文章

 

排序+DFS+剪枝+二分查找

如果最后一个元素不二分查找会超时？？

 

代码：

 1 vector<vector<int> > res;

 2 

 3 void dfs(vector<int> &num, vector<int> ans, int pos, int left, int sum) {

 4   if (left == 0) {

 5     if (sum == 0)

 6       res.push_back(ans);

 7     return;

 8   }

 9 

10   if (left == 1) {

11     int l = pos;

12     int r = num.size() - 1;

13     while (l <= r) {

14       int m = (l + r) / 2;

15       if (num[m] + sum == 0) {

16         ans.push_back(num[m]);

17         res.push_back(ans);

18         ans.pop_back();

19         return;

20       }

21       else if (num[m] + sum < 0)

22         l = m + 1;

23       else

24         r = m - 1;

25     }

26     return;

27   }

28 

29   unordered_set<int> old;

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

31     if (old.find(num[i]) != old.end())

32       continue;

33     if (sum + left * num[i] > 0)

34       break;

35     if (sum + num[i] + (left - 1 ) * num[num.size() - 1] < 0)

36       continue;

37     old.insert(num[i]);

38     ans.push_back(num[i]);

39     dfs(num, ans, i + 1, left - 1, sum + num[i]);

40     ans.pop_back();

41   }

42 }

43 

44 vector<vector<int> > threeSum(vector<int> &num) {

45   sort(num.begin(), num.end());

46   dfs(num, vector<int>(), 0, 3, 0);

47   return res;

48 }