缺失的第一个正整数(leetcode 41)

class Solution {
public:
    int firstMissingPositive(vector<int>& nums) 
    {
        if (nums.size() == 0) return 1;
        int n = nums.size();
        int p = 0;

        while (p < n)
        {
            int num = nums[p];
            if (num >= 1 && num <= n && nums[num - 1] != num)
            {
                swap(nums[p],nums[num-1]);
            }
            else
            {
                p++;
            }
        }

        for (int i = 0; i < n; i++)
            if (nums[i] != i + 1)
                return i + 1;
        return n + 1;
    }
};