rs: add «279. Perfect Squares»

Signed-off-by: Matej Focko <mfocko@redhat.com>

rs/perfect-squares.rs

@@ -0,0 +1,65 @@
+use std::cmp;
+
+// We are guaranteed ‹n ∈ ⟨1 ; 10⁴⟩›
+const UPPER_BOUND: i32 = 2000000000;
+
+// Flag for unknown values
+const UNKNOWN: i32 = -1;
+
+struct Solver {
+    dp: Vec<i32>,
+}
+
+impl Solver {
+    fn new(n: i32) -> Self {
+        let mut dp = vec![UNKNOWN; (n as usize) + 1];
+        dp[0] = 0;
+
+        Self { dp }
+    }
+
+    fn solve(&mut self, n: i32) -> i32 {
+        if n < 0 {
+            return UPPER_BOUND;
+        }
+
+        let idx = n as usize;
+        if self.dp[idx] != UNKNOWN {
+            return self.dp[idx];
+        }
+
+        self.dp[idx] = UPPER_BOUND;
+        for i in (1..=n).take_while(|i| i * i <= n) {
+            self.dp[idx] = cmp::min(self.dp[idx], 1 + self.solve(n - i * i));
+        }
+
+        self.dp[idx]
+    }
+}
+
+struct Solution {}
+impl Solution {
+    pub fn num_squares(n: i32) -> i32 {
+        Solver::new(n).solve(n)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_1() {
+        assert_eq!(Solution::num_squares(12), 3);
+    }
+
+    #[test]
+    fn test_2() {
+        assert_eq!(Solution::num_squares(13), 2);
+    }
+
+    #[test]
+    fn test_3() {
+        assert_eq!(Solution::num_squares(55), 4);
+    }
+}