LeetCode:Find the Duplicate Number

Posted 2020-07-20 walker lee

Find the Duplicate Number

Total Accepted: 29207 Total Submissions: 74613 Difficulty: Hard

Given an array nums containing n + 1 integers where each integer is between 1 and n (inclusive), 

prove that at least one duplicate number must exist. Assume that there is only one duplicate number, find the duplicate one.

Note:

1. You must not modify the array (assume the array is read only).
2. You must use only constant, O(1) extra space.
3. Your runtime complexity should be less than O(n2).
4. There is only one duplicate number in the array, but it could be repeated more than once.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

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思路：

解法类似于找有环的链表中的入口点。

c++ code:

class Solution {
public:
    int findDuplicate(vector<int>& nums) {
        
        int size = nums.size();
        
        if(size < 2) return -1;
        
        int slow = nums[0];
        int fast = nums[nums[0]];
        
        while(slow != fast) {
            slow = nums[slow];
            fast = nums[nums[fast]];
        }
        
        fast = 0;
        
        while(slow != fast) {
            slow = nums[slow];
            fast = nums[fast];
        }
        return slow;
    }
};