51nod 2499 不降的数字 dfs搜索剪枝

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/m0_37428263/article/details/89055656

点击打开链接

//dfs+剪枝
#include<bits/stdc++.h>
using namespace std;
int n,sum1=-1;
string s;
void dfs(int sol[],int cnt,int pos,int num)
{
    if(cnt==num) {
        int sum=0;
        for(int i=0;i<cnt;i++)
        {
            sum=sum*10+sol[ i ];
        }
        //cout<<sum<<endl;
        if( sum <= n ) {
                sum1=max(sum,sum1);
        }
        return;
        //cout<<sum1<<endl;
    }
    for(int i=pos;i<=9;i++)
    {
        if(i==0&&cnt==0) continue;
        if(num==s.length()) {
            sol[cnt]=i;
            if(sol[0]>s[0]+'0') continue;
            if(sol[0]==s[0]+'0') {
                if(sol[cnt]<=s[cnt]+'0')
                {
                    dfs(sol,cnt+1,i,num);
                }
                else continue;
            }
            else {
                dfs( sol,cnt+1,i,num);
            }
        }
        else {
            sol[cnt]=i;
            dfs( sol,cnt+1,i,num);
        }
    }
    return;
}
int main()
{
    scanf("%d",&n);
    if(n==0) {
        cout<<"0"<<endl;
        return 0;
    }
    else {
        s.clear();
        int k=n,num=0;
        while(k)
        {
            s+='0'+k%10;
            k/=10;
            num++;
        }
        reverse(s.begin(),s.end());
        for(int i=num;i>=1;i--)
        {
            int sol[15];
            dfs(sol,0,0,i);
            if(sum1!=-1) {
                break;
            }
        }
        cout<<sum1<<endl;
    }
    return 0;
}