[Leetcode] Find the Duplicate number

There is mainly, solutions to solve, this problem.

The first one is very clever, using the idea of the cycle detection, and runs in O (n) time. Refer to this post, whose code is rewritten below.

1 classSolution {2  Public:3     intFindduplicate (vector<int>&nums) {4         intn = nums.size (), slow = n-1, fast = N-1;5          Do {6slow = Nums[slow]-1;7Fast = Nums[nums[fast]-1] -1;8} while(Slow! =fast);9Fast = N-1; Ten          Do { Oneslow = Nums[slow]-1; AFast = Nums[fast]-1; -} while(Slow! =fast); -         returnSlow +1; the     } -};

The second one uses binary search to search for possible numbers in ranges [1, N] and eliminate one half at each Time. You could refer to this nice post for a good explanation. The code is rewritten below in C + +.

1 classSolution {2  Public:3     intFindduplicate (vector<int>&nums) {4         intn = nums.size (), L =1, R = N-1;5          while(L <r) {6             intm = L + ((r-l) >>1);7             intCNT =Notgreaterthan (Nums, m);8             if(CNT <= m) L = m +1;9             ElseR =m;Ten         }  One         returnl; A     } - Private: -     intNotgreaterthan (vector<int>& Nums,inttarget) { the         intCNT =0; -          for(intnum:nums) -             if(Num <=target) -cnt++; +         returnCNT; -     } +};

[Leetcode] Find the Duplicate number