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public class Solution {
    public int findDuplicate(int[] nums) {
        if (nums == null || nums.length < 2) return -1;
        int len = nums.length;
        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 fast;
    }
}