LeetCode/rs/perfect-squares.rs

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);
    }
}