LeetCode-in-Java

397. Integer Replacement

Medium

Given a positive integer n, you can apply one of the following operations:

1. If n is even, replace n with n / 2.
2. If n is odd, replace n with either n + 1 or n - 1.

Return the minimum number of operations needed for n to become 1.

Example 1:

Input: n = 8

Output: 3

Explanation: 8 -> 4 -> 2 -> 1

Example 2:

Input: n = 7

Output: 4

Explanation: 7 -> 8 -> 4 -> 2 -> 1 or 7 -> 6 -> 3 -> 2 -> 1

Example 3:

Input: n = 4

Output: 2

Constraints:

• 1 <= n <= 231 - 1

Solution

import java.util.HashMap;
import java.util.Map;

public class Solution {
    public int integerReplacement(int n) {
        Map<Integer, Integer> dp = new HashMap<>();
        return solve(n, dp);
    }

    private int solve(int n, Map<Integer, Integer> dp) {
        if (n == 1) {
            return 0;
        }
        if (dp.containsKey(n)) {
            return dp.get(n);
        }
        int ans;
        if (n % 2 == 0) {
            ans = 1 + solve(n >>> 1, dp);
        } else {
            ans = 1 + Math.min(solve(n + 1, dp), solve(n - 1, dp));
        }
        dp.put(n, ans);
        return ans;
    }
}

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