leetcode-First Missing Positive

https://leetcode.com/problems/first-missing-positive/#/description

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.

题解：这道题目比较简单，需要注意的是几个陷阱：数组为空，数组里数据重复。

比较有技巧的地方是O(1)space，因为考虑到数据重复，此处只能排序， 这就要求重用nums数组空间。

引用网上一位网友的题解：

Put each number in its right place.

For example:

When we find 5, then swap it with A[4].

At last, the first place where its number is not right, return the place + 1.

程序：

class Solution {
public:
int firstMissingPositive(vector<int>& nums) {
for (int i = 0; i < nums.size(); i++)
{
while (nums[i] > 0 && nums[i] <= nums.size() && nums[i] != nums[nums[i] -1])
{
swap(nums[i], nums[nums[i] -1]);
}
}
for (int i = 0; i < nums.size(); i++)
{
if (nums[i] != i + 1)
{
return i + 1;
}
}
return nums.size() + 1;
}
};