fzu 2109 Mountain Number

题目链接：http://acm.fzu.edu.cn/problem.php?pid=2109

题目大意：

设x=a[0]a[1]a[2]....a[len-1],对任意a[2*i+1]大于a[2*i]和a[2*i+2]。

求L~R（1<=L<=R<=10^9）中有多少个满足条件的x。

题目思路：

第一道数位dp。

dfs(i,s,p,first,flag);

第i位(包括前导0)，前一位数为s，p表示是否为峰顶，first表示是否是开头，flag表示是否是前缀。

代码：

#include <stdlib.h>#include <string.h>#include <stdio.h>#include <ctype.h>#include <math.h>#include <stack>#include <queue>#include <map>#include <set>#include <vector>#include <string>#include <iostream>#include <algorithm>using namespace std;#define ll __int64#define ls rt<<1#define rs ls|1#define lson l,mid,ls#define rson mid+1,r,rs#define middle (l+r)>>1#define clr_all(x,c) memset(x,c,sizeof(x))#define clr(x,c,n) memset(x,c,sizeof(x[0])*(n+1))#define eps (1e-8)#define MOD 1000000007#define INF 0x3f3f3f3f#define PI (acos(-1.0))#pragma comment(linker, "/STACK:102400000,102400000")template <class T> T _max(T x,T y){return x>y? x:y;}template <class T> T _min(T x,T y){return x<y? x:y;}template <class T> T _abs(T x){return (x < 0)? -x:x;}template <class T> T _mod(T x,T y){return (x > 0)? x%y:((x%y)+y)%y;}template <class T> void _swap(T &x,T &y){T t=x;x=y;y=t;}template <class T> void getmax(T& x,T y){x=(y > x)? y:x;}template <class T> void getmin(T& x,T y){x=(x<0 || y<x)? y:x;}int TS,cas=1;const int M=10+5;int l,r;int num[M],len;int f[M][M][2];int dfs(int i,int s,bool p,bool first,bool flag){if(i==-1) return 1;if(!flag && ~f[i][s][p]) return f[i][s][p];int res=0,u=(flag? num[i]:9);for(int j=0;j<=u;j++){if(first && !j) res+=dfs(i-1,9,0,1,0);else if(p && j>=s) res+=dfs(i-1,j,p^1,0,flag&&(j==u));else if(!p && j<=s) res+=dfs(i-1,j,p^1,0,flag&&(j==u));}return flag? res:f[i][s][p]=res;}int cal(int n){clr_all(f,-1);for(len=0;n;) num[len++]=n%10,n/=10;return dfs(len-1,9,0,1,1);}void run(){int i,j;scanf("%d%d",&l,&r);printf("%d\n",cal(r)-cal(l-1));}void preSof(){}int main(){    //freopen("input.txt","r",stdin);    //freopen("output.txt","w",stdout);    preSof();    //run();    //while((~scanf("%d",&n))) run();    for(scanf("%d",&TS);cas<=TS;cas++) run();    return 0;}