287.find-the-duplicate-number.cpp

class Solution {
public:
    int findDuplicate(vector<int>& nums) {
        for (int i = 0; i < nums.size(); i++) {
            if (nums[i] != i + 1 && nums[i] != nums[nums[i] - 1]) {
                swap(nums[i], nums[nums[i] - 1]);
                i--;
            }
        }
        for (int i = 0; i < nums.size(); i++) {
            if (nums[i] != i + 1) {
                return nums[i];
            }
        }
        return -1;
    }
};