41

41.first-missing-positive.rb

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+# -*- coding: utf-8 -*-
+#
+# @lc app=leetcode id=41 lang=ruby
+#
+# [41] First Missing Positive
+#
+# https://leetcode.com/problems/first-missing-positive/description/
+#
+# 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.
+
+
+# O(n) space.
+
+# @param {Integer[]} nums
+# @return {Integer}
+def first_missing_positive(nums)
+  s = nums.size
+  xs = Array.new(s+1)
+  nums.each do |n|
+    xs[n] = n if n > 0 && n <= s
+  end
+  (1..s).each do |i|
+    return i if xs[i].nil?
+  end
+  xs.size
+end
+
+
+# O(1) space.
+
+def first_missing_positive(nums)
+  s = nums.size
+  (0..s-1).each do |i|
+    while nums[i] > 0 && nums[i] <= s && nums[nums[i]-1] != nums[i]
+      nums[nums[i]-1], nums[i] = nums[i], nums[nums[i]-1]
+    end
+  end
+  (0..s-1).each do |i|
+    return i+1 if nums[i] != i+1
+  end
+  s+1
+end