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  • Codeforces-Two Buttons-520problemB(思维题)

    B. Two Buttons

    Vasya has found a strange device. On the front panel of a device there are: a red button, a blue button and a display showing some positive integer. After clicking the red button, device multiplies the displayed number by two. After clicking the blue button, device subtracts one from the number on the display. If at some point the number stops being positive, the device breaks down. The display can show arbitrarily large numbers. Initially, the display shows number n.

    Bob wants to get number m on the display. What minimum number of clicks he has to make in order to achieve this result?

    Input

    The first and the only line of the input contains two distinct integers n and m (1 ≤ n, m ≤ 104), separated by a space .

    Output

    Print a single number — the minimum number of times one needs to push the button required to get the number m out of number n.

    Examples
    input
    4 6
    output
    2
    input
    10 1
    output
    9
    Note

    In the first example you need to push the blue button once, and then push the red button once.

    In the second example, doubling the number is unnecessary, so we need to push the blue button nine times.

    两种思路,数学方法算出结果or暴力bfs

    以下为数学算法(逆推思路)

    if n>m ans=ans+n-m

    if n<=m 

    if m为奇数 m+1后再除2

    if m为偶数 m直接除2

    若n==m ans=ans

    若n>m ans=ans+n-m

    代码如下

     1 #include <iostream>
     2 using namespace std;
     3 int a[10000000];
     4 int main(){
     5     int n,m,ans(0),j(0);
     6     cin>>n>>m;
     7     if(n>m){
     8         cout<<n-m;
     9         return 0;
    10     }
    11     a[m]++;
    12     while(m!=n){
    13         if(n>m){
    14             cout<<ans+n-m;
    15             return 0;
    16         }
    17         if(m%2==0&&a[m/2]==0) {
    18             m=m/2;
    19             a[m]++;
    20             ans++;
    21         }
    22         else {
    23             m++;
    24             a[m]++;
    25             ans++;
    26         }
    27     }
    28     cout<<ans<<'
    ';
    29 }
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  • 原文地址:https://www.cnblogs.com/Never-Land/p/11139962.html
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