Solve first-missing-positive.

source/first_missing_positive/mod.rs

@@ -0,0 +1,36 @@
+pub fn first_missing_positive(mut numbers: Vec<i32>) -> i32 {
+  let numbers_length = numbers.len();
+
+  // Mark all numbers below 1 and above the maximum range as i32::MAX.
+  for index in 0..numbers_length {
+    if numbers[index] < 1 || numbers[index] > numbers_length as i32 {
+      numbers[index] = i32::MAX;
+    }
+  }
+
+  for index in 0..numbers_length {
+    // For each index get its number and subtract 1.
+    let index = numbers[index].abs() as usize - 1;
+
+    // If that index then isn't i32::MAX - 1 (ie. negative or above range).
+    if index as i32 != i32::MAX - 1 {
+      // Then mark the number for that index as negative.
+      numbers[index] = -numbers[index].abs();
+    }
+  }
+
+  // Then the first positive number we find, the index of that number will be
+  // the first missing positive. And if there are only negative numbers, then
+  // the next positive number is the total amount of numbers + 1.
+  numbers
+    .into_iter()
+    .enumerate()
+    .find_map(|(index, number)| {
+      if number.is_positive() {
+        Some(index + 1)
+      } else {
+        None
+      }
+    })
+    .unwrap_or_else(|| numbers_length + 1) as i32
+}

source/lib.rs

@@ -3,6 +3,7 @@ pub mod contains_duplicate;
 pub mod contains_duplicate_ii;
 pub mod excel_sheet_column_number;
 pub mod excel_sheet_column_title;
+pub mod first_missing_positive;
 pub mod fizz_buzz;
 pub mod implement_strstr;
 pub mod integer_to_roman;

tests/first_missing_positive.rs

@@ -0,0 +1,14 @@
+use leetcode::first_missing_positive::first_missing_positive;
+
+use test_case::test_case;
+
+#[test_case(&[1, 2, 0], 3; "example 1")]
+#[test_case(&[3, 4, -1, 1], 2; "example 2")]
+#[test_case(&[7, 8, 9, 11, 12], 1; "example 3")]
+#[test_case(&[1, 3, 2], 4; "maximum")]
+#[test_case(&[1, 3, 3], 2; "duplicate")]
+#[test_case(&[0], 1; "zero")]
+#[test_case(&(0..=500000).into_iter().collect::<Vec<i32>>(), 500001; "massive")]
+fn test_first_missing_positive(input: &[i32], expected: i32) {
+  assert_eq!(first_missing_positive(input.to_vec()), expected);
+}