題目

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.

    如果把該array中的n放在第n個位置上，那麼遍歷一遍後，第一個位置上不符合的就是要求的值

class Solution(object):
    def firstMissingPositive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        L=len(nums)
        for m in range(L):
            if nums[m]>L:
                continue
            val = nums[m]-1
            while nums[m]>0 and val<L and nums[val] != nums[m]:
                nums[m],nums[val]=nums[val],nums[m]
                val = nums[m]-1
        for m in range(len(nums)):
            if nums[m] != m +1:
                return m +1
        return L+1