Find the Duplicate Number

Source

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

Java

//O(nlogn)
public class Solution {
    public int findDuplicate(int[] nums) {
        int left=1;
        int right=nums.length-1;

        while(left<=right){
            int mid = (left+right)/2;
            int count=0;
            for(int num: nums){
                if(num<=mid){
                    count++;
                }
            }
            if(count>mid){
                right=mid-1;
            }
            else{
                left=mid+1;
            }
        }
        return left;
    }
}

//O(n)
//跟"linked list cycle II"算法一样
public class Solution {
    public int findDuplicate(int[] nums){
        int slow=0;
        int fast=0;
        while(true){
            slow=nums[slow];
            fast=nums[nums[fast]];
            if(slow==fast){
                break;
            }
        }
        int finder=0;
        while(finder!=slow){
            slow=nums[slow];
            finder=nums[finder];
        }
        return slow;
    }
}s

Reference

Find the Duplicate Number书影博客