[Leetcode]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.

找到第一个丢失的正整数~思路用到了Counting sort~可以利用数组的index来作为数字本身的索引，把正数按照递增顺序依次放到数组中，即让A[0]=1, A[1]=2, A[2]=3 , 最后如果哪个数组元素不满足A[i]=i+1就说明i+1就是第一个缺失的正数~对于不在范围内的数字，比如说负数，0，或者超过数组长度的正数，可以直接跳过~这题之前做过一次还是不太熟啊~对Counting
Sort之前也不是很熟悉~

class Solution:
# @param A, a list of integers
# @return an integer
def firstMissingPositive(self, A):
if A is None or len(A) == 0:
return 1
for i in xrange(len(A)):
while A[i] > 0 and A[i] <= len(A) and A[A[i] - 1] != A[i]:
A[A[i] - 1], A[i] = A[i], A[A[i] - 1]
for i in xrange(len(A)):
if A[i] != i + 1:
return i + 1
return len(A) + 1