First Missing Positive

41. First Missing Positive

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

Example 1:

Input: [1,2,0]
Output: 3

Example 2:

Input: [3,4,-1,1]
Output: 2

Example 3:

Input: [7,8,9,11,12]
Output: 1

Note:

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

解题思路：

第一步：将数组中大于0的整数做一个hash映射，同时获取最大值ARRMAX。

第二步：从1到ARRMAX遍历hash表，找到第一个为0的终止，返回最后查找索引值即可。

 

 

#define HASHNUM 100000

int firstMissingPositive(int* nums, int numsSize)
{
    int numsMax = 0;
    int hash[HASHNUM] = {0};
    // hash and seek for array max num.
    for(int idx = 0; idx < numsSize; ++idx)
    {
        if(nums[idx] <= 0 ) continue;
        else
        {
            hash[nums[idx]] = 1;
            if(numsMax < nums[idx])
            {
                numsMax = nums[idx];
            }
        }
    }

    // judge for missing
    int retIdx = 1;
    for(retIdx = 1; retIdx <= numsMax; ++retIdx)
    {
        if(hash[retIdx] == 0)
        {
            break;
        }
    }

    return retIdx;
}