LC 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.

 

 

Runtime: 4 ms, faster than 100.00% of C++ online submissions for Broken Calculator.

Memory Usage: 4.7 MB, less than 100.00% of C++ online submissions for Broken Calculator.

class Solution {
public:
  int brokenCalc(int X, int Y) {
    if(X == Y) return 0;
    if(X < Y) {
      if(Y&1 == 1) {
        int tmp = ((Y >> 1)+1) << 1;
        return brokenCalc(X,tmp >> 1) + tmp - Y + 1;
      }else {
        return brokenCalc(X,Y>>1)+1;
      }
    }
    return X - Y;
  }
};