991. Broken Calculator

On a broken calculator that has a number showing on its display, we can perform two operations:

• Double: Multiply the number on the display by 2, or;
• Decrement: Subtract 1 from the number on the display.

Initially, the calculator is displaying the number X.

Return the minimum number of operations needed to display the number Y.

 

Example 1:

Input: X = 2, Y = 3
Output: 2
Explanation: Use double operation and then decrement operation {2 -> 4 -> 3}.

Example 2:

Input: X = 5, Y = 8
Output: 2
Explanation: Use decrement and then double {5 -> 4 -> 8}.

Example 3:

Input: X = 3, Y = 10
Output: 3
Explanation:  Use double, decrement and double {3 -> 6 -> 5 -> 10}.

Example 4:

Input: X = 1024, Y = 1
Output: 1023
Explanation: Use decrement operations 1023 times.

 

Note:

1. 1 <= X <= 10^9
2. 1 <= Y <= 10^9

class Solution {
    public int brokenCalc(int X, int Y) {
            if(X == Y) return 0;
            if(Y < X) return X-Y;
            return 1 + (Y % 2 == 0 ? brokenCalc(X, Y/2) : brokenCalc(X, Y + 1));
        }
}

草了，居然可以反过来想，intuitively如何把y变成x。

class Solution {
    public int brokenCalc(int x, int y) {
        int res = 0;
        while(y > x) {
            y = (y % 2 == 0) ? y / 2 : y + 1;
            res++;
        }
        return res + x - y;
    }
}