375. Guess Number Higher or Lower II

We are playing the Guessing Game. The game will work as follows:

  1. I pick a number between 1 and n.
  2. You guess a number.
  3. If you guess the right number, you win the game.
  4. If you guess the wrong number, then I will tell you whether the number I picked is higher or lower, and you will continue guessing.
  5. Every time you guess a wrong number x, you will pay x dollars. If you run out of money, you lose the game.

Given a particular n, return the minimum amount of money you need to guarantee a win regardless of what number I pick.

 

Example 1:

Input: n = 10
Output: 16
Explanation: The winning strategy is as follows:
- The range is [1,10]. Guess 7.
    - If this is my number, your total is $0. Otherwise, you pay $7.
    - If my number is higher, the range is [8,10]. Guess 9.
        - If this is my number, your total is $7. Otherwise, you pay $9.
        - If my number is higher, it must be 10. Guess 10. Your total is $7 + $9 = $16.
        - If my number is lower, it must be 8. Guess 8. Your total is $7 + $9 = $16.
    - If my number is lower, the range is [1,6]. Guess 3.
        - If this is my number, your total is $7. Otherwise, you pay $3.
        - If my number is higher, the range is [4,6]. Guess 5.
            - If this is my number, your total is $7 + $3 = $10. Otherwise, you pay $5.
            - If my number is higher, it must be 6. Guess 6. Your total is $7 + $3 + $5 = $15.
            - If my number is lower, it must be 4. Guess 4. Your total is $7 + $3 + $5 = $15.
        - If my number is lower, the range is [1,2]. Guess 1.
            - If this is my number, your total is $7 + $3 = $10. Otherwise, you pay $1.
            - If my number is higher, it must be 2. Guess 2. Your total is $7 + $3 + $1 = $11.
The worst case in all these scenarios is that you pay $16. Hence, you only need $16 to guarantee a win.

Example 2:

Input: n = 1
Output: 0
Explanation: There is only one possible number, so you can guess 1 and not have to pay anything.

Example 3:

Input: n = 2
Output: 1
Explanation: There are two possible numbers, 1 and 2.
- Guess 1.
    - If this is my number, your total is $0. Otherwise, you pay $1.
    - If my number is higher, it must be 2. Guess 2. Your total is $1.
The worst case is that you pay $1.

 

Constraints:

  • 1 <= n <= 200

Rust Solution

struct Solution;
use std::collections::HashMap;

impl Solution {
    fn get_money_amount(n: i32) -> i32 {
        let mut memo: HashMap<(i32, i32), i32> = HashMap::new();
        Self::dp(1, n, &mut memo)
    }

    fn dp(left: i32, right: i32, memo: &mut HashMap<(i32, i32), i32>) -> i32 {
        if left == right {
            0
        } else {
            if let Some(&res) = memo.get(&(left, right)) {
                return res;
            }
            let mut res = std::i32::MAX;
            for i in left..right {
                let a = if i != left {
                    Self::dp(left, i - 1, memo)
                } else {
                    0
                };
                let b = if i != right {
                    Self::dp(i + 1, right, memo)
                } else {
                    0
                };
                res = res.min(i + a.max(b));
            }
            memo.insert((left, right), res);
            res
        }
    }
}

#[test]
fn test() {
    let n = 10;
    let res = 16;
    assert_eq!(Solution::get_money_amount(n), res);
}

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