287. Find the Duplicate Number

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.

一刷
题解：由于数组长度为n+1，且数组中的元素的大小在[1, n]
思路同142， 快慢指针。
duplicate: entry of the circle
meet place: circle

class Solution {
    public int findDuplicate(int[] nums) {
        if(nums.length<1) return -1;
        int slow = nums[0];
        int fast = nums[nums[0]];
        while(slow!=fast){
            slow = nums[slow];
            fast = nums[nums[fast]];
        }
        
        fast = 0;
        while(fast!=slow){
            fast = nums[fast];
            slow = nums[slow];
        }
        
        return fast;
    }
}