Mountain Number （数位dp）

One integer number x is called "Mountain Number" if:

(1) x>0 and x is an integer;

(2) Assume x=a[0]a[1]...a[len-2]a[len-1](0≤a[i]≤9, a[0] is positive). Any a[2i+1] is larger or equal to a[2i] and a[2i+2](if exists).

For example, 111, 132, 893, 7 are "Mountain Number" while 123, 10, 76889 are not "Mountain Number".

Now you are given L and R, how many "Mountain Number" can be found between L and R (inclusive) ?

Input

The first line of the input contains an integer T (T≤100), indicating the number of test cases.

Then T cases, for any case, only two integers L and R (1≤L≤R≤1,000,000,000).

Output

For each test case, output the number of "Mountain Number" between L and R in a single line.

Sample Input

31 101 1001 1000

Sample Output

954384

题目大概：

找出像山一样的数的个数，即偶数位比奇数位大的数，找出范围内这种数的个数。

思路：

基础数位dp，一要注意前导零，二要分好类，讨论位数的大小。

代码：

#include <iostream>#include <cstring>#include <cstdio>using namespace std;long long a[22];long long dp[22][25][2];long long dpp(int pos,int qian,int jo,int lead,int limit){    if(pos==-1)return 1;    if(!limit&&dp[pos][qian][jo]!=-1)return dp[pos][qian][jo];    long long sun=0;    int end=limit?a[pos]:9;    for(int i=0;i<=end;i++)    {        if(!(lead||i))        sun+=dpp(pos-1,9,0,0,limit&&i==a[pos]);        else if(jo&&i>=qian)        sun+=dpp(pos-1,i,!jo,1,limit&&i==a[pos]);        else if(!jo&&i<=qian)        sun+=dpp(pos-1,i,!jo,1,limit&&i==a[pos]);    }    if(!limit)dp[pos][qian][jo]=sun;    return sun;}long  long go(long long x){    int pos=0;    while(x)    {        a[pos++]=x%10;        x/=10;    }    return dpp(pos-1,9,0,0,1);}int main(){    long long n,m,t;    memset(dp,-1,sizeof(dp));    scanf("%I64d",&t);    while(t--)    {   scanf("%I64d%I64d",&n,&m);        printf("%I64d\n",go(m)-go(n-1));    }    return 0;}