原题地址：https://oj.leetcode.com/problems/first-missing-positive/

题意：

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.

解题思路：要求时间复杂度为O(n)和常数空间。又是一个tricky的解法。

class Solution:    # @param A, a list of integers    # @return an integer    # @a very subtle solution    def firstMissingPositive(self, A):        n=len(A)        for i in range(n):            if A[i]<=0: A[i]=n+2        for i in range(n):            if abs(A[i])<=n:                curr=abs(A[i])-1                A[curr]=-abs(A[curr])        for i in range(n):            if A[i]>0: return i+1        return n+1