math: Add find_max_recursion

math/src/find_max_recursion.rs

@@ -0,0 +1,48 @@
+// Copyright (c) 2023 Xu Shaohua <shaohua@biofan.org>. All rights reserved.
+// Use of this source is governed by General Public License that can be found
+// in the LICENSE file.
+
+/// Find max value in list
+///
+/// Divide and Conquer algorithm
+#[must_use]
+pub const fn find_max(nums: &[i32], left: usize, right: usize) -> i32 {
+    debug_assert!(!nums.is_empty());
+    debug_assert!(left < nums.len());
+    debug_assert!(right < nums.len());
+    if left == right {
+        return nums[left];
+    }
+    // the middle
+    let mid = (left + right) >> 1;
+    // find max in range[left, mid]
+    let left_max = find_max(nums, left, mid);
+    // find max in range[mid + 1, right]
+    let right_max = find_max(nums, mid + 1, right);
+
+    if left_max >= right_max {
+        left_max
+    } else {
+        right_max
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::find_max;
+
+    #[test]
+    fn test_find_max() {
+        for nums in &[[3, 2, 1], [-3, -2, -1], [3, -3, 0], [30, 31, 29]] {
+            assert_eq!(
+                find_max(nums, 0, nums.len() - 1),
+                *nums.iter().max().unwrap()
+            );
+        }
+        let nums = [1, 3, 5, 7, 9, 2, 4, 6, 8, 10];
+        assert_eq!(
+            find_max(&nums, 0, nums.len() - 1),
+            *nums.iter().max().unwrap()
+        );
+    }
+}

math/src/lib.rs

@@ -17,6 +17,7 @@ pub mod digits;
 pub mod factorial;
 pub mod factors;
 pub mod fibonacci;
+pub mod find_max_recursion;
 pub mod find_min_recursion;
 pub mod floor;
 pub mod gcd;