First Missing Positive

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.

 

Subscribe to see which companies asked this question

Show Tags

Show Similar Problems

 

class Bitmap {
public:
    Bitmap() {
        int i = 0;
        while (i < size) {
            bitmap[i] = 0;
            i++;
        }
    }

/*

* 设置第i位

* i >> SHIFT 相当于 i / (2 ^ SHIFT),

* i&MASK相当于mod操作 m mod n 运算

*/

    void bitset(int i) {

        bitmap[i >> SHIFT] |= 1 << (i & MASK);

    }

//获取第i位

    int bittest(int i) {

        return bitmap[i >> SHIFT] & (1 << (i & MASK));


    }

//清除第i位

    int bitclear(int i) {

        return bitmap[i >> SHIFT] & ~(1 << (i & MASK));

    }
private:

    const static int INT_BITS  = sizeof(int);

    const static int SHIFT =  5; // 2^5=32

    const static int  MASK = 0x1f; // 2^5=32

    const static int  _MAX  = 1024;//max number
    const static int size = _MAX / INT_BITS;
    int bitmap[size];
};

class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        int size_num = nums.size();
        int i;
        int positive_num = 0;
        Bitmap bitmap;
        
        for (i=0; i<size_num; i++) {
            if (nums[i] > 0 && bitmap.bittest(nums[i]) == 0) {
                positive_num++;
                bitmap.bitset(nums[i]);
            }
        }
        
        for (i=1; i<=positive_num; i++) {
            if (bitmap.bittest(i) == 0) {
                return i;
            }
        }
        return i;
    }
};