[LeetCode] 030: First Missing Positive

[Problem]

Given an unsorted integer array, find the first missing positive integer.

For example,
Given [1,2,0] return 3,
and [3,4,-1,1] return 2.

Your algorithm should run in O(n) time and uses constant space.

[Solution]

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        for(int i = 0; i < n; ++i){
            if(A[i] <= 0 || A[i] == i+1)continue;
            int val = A[i], tmpV;
            while(val > 0 && val <= n && val != A[val-1]){
                tmpV = A[val-1];
                A[val-1] = val;
                val = tmpV;
            }
        }
        
        // find
        for(int i = 0; i < n; ++i){
            if(A[i] != i+1){
                return i+1;
            }
        }
        return n+1;
    }
};

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