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41. First Missing Positive
Hard
Given an unsorted integer array nums, return the smallest missing positive integer.
You must implement an algorithm that runs in O(n) time and uses constant extra space.
Example 1:
Input: nums = [1,2,0]
Output: 3
Example 2:
Input: nums = [3,4,-1,1]
Output: 2
Example 3:
Input: nums = [7,8,9,11,12]
Output: 1
Constraints:
1 <= nums.length <= 5 * 105-231 <= nums[i] <= 231 - 1
Solution
# @param {Integer[]} nums
# @return {Integer}
def first_missing_positive(nums)
nums.each_with_index do |num, i|
next if num <= 0 || num > nums.length || num == i + 1
dfs(nums, num)
end
nums.each_with_index do |num, i|
return i + 1 if num != i + 1
end
nums.length + 1
end
private
def dfs(nums, val)
return if val <= 0 || val > nums.length || val == nums[val - 1]
temp = nums[val - 1]
nums[val - 1] = val
dfs(nums, temp)
end