[LeetCode]题解（python）：041-First Missing Positive

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题目来源

https://leetcode.com/problems/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.

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题意分析

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Input:a list with many numbers

Output:the first missing positive number

Conditions:最小的missing 正整数，注意要0（n）级别的算法

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题目思路

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利用字典去做，因为字典查询是0（n）的，不过会使用0（n）的空间，AC了，但是网上说要用triky，可能是leecode评测放宽了……以后再回来看看

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AC代码（Python）

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_author_ = "YE"
# -*- coding:utf-8 -*-

class Solution(object):
    def firstMissingPositive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        len1 = len(nums)
        if len1 == 0:
            return 1
        dic = {}
        for num in nums:
            if num > 0:
                dic[num] = num
        for i in range(1, len1 + 1):
            if dic.get(i, -1) == -1:
                return i
        return len1 + 1