[leetcode]First Missing Positive @ Python

时间：2014-06-09 17:44:43 收藏：0 阅读：980

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

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