LeetCode – Refresh – First Missing Posititive

Two methods:

1. Use extra memory:

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        unordered_set<int> sets;
        int result = 1;
        for (int i = 0; i < n; i++) sets.insert(A[i]);
        while (sets.find(result++) != sets.end());
        return result-1;
    }
};

 

2. In place:

The trick is the relationship between value and index. Since it is first missing positive number, if the number is not missing, it should satisfy index + 1 = value in [1, n].

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        if (n == 0) return 1;
        int i = 0;
        while (i < n) {
            if (A[i] > 0 && A[i] <= n && A[i] != i+1 && A[A[i]-1] != A[i]) {
                [](int &a, int &b){int t = a; a = b; b = t;}(A[i], A[A[i]-1]);
                continue;
            }
            i++;
        }
        for (i = 0; i < n; i++) {
            if (A[i] != i+1) break;
        }
        return i+1;
    }
};