[LintCode] 丢失的第一个正整数

class Solution {
public:
    /**    
     * @param A: a vector of integers
     * @return: an integer
     */
    int firstMissingPositive(vector<int> A) {
        // write your code here
        int n = A.size();
        for (int i = 0; i < n; i++)
            while (A[i] > 0 && A[i] <= n && A[A[i] - 1] != A[i])
                swap(A[i], A[A[i] - 1]);
        for (int i = 0; i < n; i++)
            if (A[i] != i + 1)
                return i + 1;
        return n + 1;
    }
};