P67: bottom-to-top dynamic programming.

src/solutions/maximum_path_sum_i.rs

@@ -14,7 +14,6 @@ pub fn solve() -> i64 {
             triangle[row][col] += std::cmp::max(triangle[row + 1][col], triangle[row + 1][col + 1]);
         }
     }
-
     let result = triangle[0][0];
 
     assert_eq!(result, 1074);

src/solutions/maximum_path_sum_ii.rs

@@ -1,50 +1,20 @@
 use std::io::BufRead;
 
-struct CacheMap {
-    map: std::collections::HashMap<(usize, usize), i32>,
-    triangle: Vec<Vec<i32>>,
-}
-impl CacheMap {
-    fn new(triangle: Vec<Vec<i32>>) -> CacheMap {
-        CacheMap {
-            map: std::collections::HashMap::new(),
-            triangle,
-        }
-    }
-    /// Find the maximum path sum to the bottom, starting from the given row
-    /// and column indices.
-    ///
-    /// * `key` - Tuple of row and column indices.
-    ///
-    /// -> Maximum path sum.
-    fn get(&mut self, key: (usize, usize)) -> i32 {
-        let (row, col) = key;
-        if row >= self.triangle.len() {
-            return 0;
-        }
-        match self.map.get(&key) {
-            Some(&value) => value,
-            None => {
-                let left = self.get((row + 1, col));
-                let right = self.get((row + 1, col + 1));
-                let value = self.triangle[row][col] + std::cmp::max(left, right);
-                self.map.insert(key, value);
-                value
-            }
-        }
-    }
-}
-
 pub fn solve() -> i64 {
     let fhandle = std::fs::File::open("res/maximum_path_sum_ii.txt").unwrap();
     let reader = std::io::BufReader::new(fhandle);
-    let triangle: Vec<Vec<i32>> = reader
+    let mut triangle: Vec<Vec<i32>> = reader
         .lines()
         .map(|line| line.unwrap().split(' ').map(|s| s.parse().unwrap()).collect())
         .collect();
 
-    let mut cache_map = CacheMap::new(triangle);
-    let result = cache_map.get((0, 0));
+    // Bottom-up dynamic programming.
+    for row in (0..triangle.len()).rev().skip(1) {
+        for col in 0..triangle[row].len() {
+            triangle[row][col] += std::cmp::max(triangle[row + 1][col], triangle[row + 1][col + 1]);
+        }
+    }
+    let result = triangle[0][0];
 
     assert_eq!(result, 7273);
     result as i64