41. First Missing Positive.py

'''
Given an unsorted integer array, find the smallest missing positive integer.

Example 1:
Input: [1,2,0]
Output: 3

Example 2:
Input: [3,4,-1,1]
Output: 2

Example 3:
Input: [7,8,9,11,12]
Output: 1

Note:
Your algorithm should run in O(n) time and uses constant extra space.
'''

class Solution:
    def firstMissingPositive(self, nums):
        n = len(nums)
        for i in range(n):
            digit = nums[i]
            while(digit>0 and digit<=n and nums[digit-1] != digit):
                nums[i],nums[digit-1]=nums[digit-1],nums[i]
                digit = nums[i]
        for i in range(n):
            if nums[i]!=i+1:
                return i+1
        return n+1