Create first_missing_positive.py

data_structures/arrays/first_missing_positive.py

@@ -0,0 +1,74 @@
+def first_missing_positive(nums: list[int]) -> int:
+    """
+    Given an unsorted integer array nums, returns the smallest missing positive integer.
+
+    The algorithm must run in O(n) time and use O(1) auxiliary space.
+
+    Hints:
+    - Try to place each number in its correct position, so that nums[i] equals i + 1.
+    - Iterate through the array to find the first position where nums[i] != i + 1,
+      indicating the missing positive integer.
+
+    :param nums: The input unsorted integer array.
+    :return: The smallest missing positive integer.
+
+    Examples:
+    >>> first_missing_positive([1, 2, 0])
+    3
+
+    >>> first_missing_positive([3, 4, -1, 1])
+    2
+
+    >>> first_missing_positive([7, 8, 9, 11, 12])
+    1
+
+    >>> first_missing_positive([])
+    1
+
+    >>> first_missing_positive([0])
+    1
+
+    >>> first_missing_positive([1])
+    2
+
+    >>> first_missing_positive([0.9, 1.5, -2.3, 3.0, 2.7])
+    1
+
+    >>> first_missing_positive([-1, -2, -3])
+    1
+
+    >>> first_missing_positive([1.5, -2.3, 3.0, 2.7])
+    1
+
+    >>> first_missing_positive("ABC")
+    Traceback (most recent call last):
+    ...
+    TypeError: Input must be a list of numbers
+    """
+    if not isinstance(nums, list):
+        raise TypeError("Input must be a list of numbers")
+
+    # Filter out non-positive numbers and fractional parts
+    nums = [int(x) for x in nums if x > 0 and x == int(x)]
+
+    n = len(nums)
+
+    # Move each number to its correct position
+    for i in range(n):
+        while 1 <= nums[i] <= n and nums[nums[i] - 1] != nums[i]:
+            nums[nums[i] - 1], nums[i] = nums[i], nums[nums[i] - 1]
+
+    # Find the first missing positive integer
+    for i in range(n):
+        if nums[i] != i + 1:
+            return i + 1
+
+    # If all integers from 1 to n are present, return n + 1
+    return n + 1
+
+
+# Run the doctests
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()