0287. Find the Duplicate Number (M)

题目

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.

Example 1:

Input: [1,3,4,2,2]
Output: 2

Example 2:

Input: [3,1,3,4,2]
Output: 3

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($n^2$).
4. There is only one duplicate number in the array, but it could be repeated more than once.

题意

思路

链表循环查找：参考 官方解答

二分查找：参考 [LeetCode] 287. Find the Duplicate Number 寻找重复数

代码实现

Java

链表循环

class Solution {
    public int findDuplicate(int[] nums) {
        int slow = nums[0], fast = nums[0];
        do {
            slow = nums[slow];
            fast = nums[nums[fast]];
        } while (slow != fast);
        fast = nums[0];
        while (fast != slow) {
            slow = nums[slow];
            fast = nums[fast];
        }
        return slow;
    }
}

二分查找

class Solution {
    public int findDuplicate(int[] nums) {
        int left = 1, right = nums.length - 1;
        while (left < right) {
            int mid = (right - left) / 2 + left;
            int count = 0;
            for (int num : nums) {
                if (num <= mid) {
                    count++;
                }
            }
            if (count <= mid) {
                left = mid + 1;
            } else {
                right = mid;
            }
        }
        return left;
    }
}