First Missing Positive

Posted 2020-11-07 peki10

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。

 

 

 1 #define HASHNUM 100000
 2 
 3 int firstMissingPositive(int* nums, int numsSize)
 4 {
 5     int numsMax = 0;
 6     int hash[HASHNUM] = {0};
 7     // hash and seek for array max num.
 8     for(int idx = 0; idx < numsSize; ++idx)
 9     {
10         if(nums[idx] <= 0 ) continue;
11         else
12         {
13             hash[nums[idx]] = 1;
14             if(numsMax < nums[idx])
15             {
16                 numsMax = nums[idx];
17             }
18         }
19     }
20 
21     // judge for missing
22     int retIdx = 1;
23     for(retIdx = 1; retIdx <= numsMax; ++retIdx)
24     {
25         if(hash[retIdx] == 0)
26         {
27             break;
28         }
29     }
30 
31     return retIdx;
32 }