public int duplicateInArray(int[] nums) {
if (nums == null || nums.length < 2) return 0;
//模仿环形链表
int f = 0, s = 0;
while (f == 0 || f != s) {
f = nums[nums[f]];
s = nums[s];
}
f = 0;
while (f != s) {
f = nums[f];
s = nums[s];
}
return f;
}