SDSC 2017 Day 4 T3 Lift

//考场上写的30分可爱小暴力 #include<cstdio>#include<cmath>#include<iostream>using namespace std;const int moder = 1000000007;int n, a, b, k;int ans;void dfs(int na, int step){    if(step == k+1){        ans %= moder;        ans++;        return;    }    int delta = abs(na - b) - 1;    if(delta == 0)  return;    int l = na - delta;    int r = na + delta;    if(l < 1)   l = 1;    if(r > n)   r = n;    for(int i = l; i <= r; i++)        if(i != na) dfs(i, step+1);}int main(){    freopen("lift.in", "r", stdin);    freopen("lift.out", "w", stdout);    scanf("%d%d%d%d", &n, &a, &b, &k);    if(abs(a-b) == 1){        printf("0\n");        return 0;    }    dfs(a, 1);    printf("%d\n", ans);    return 0;}

//正解DP//dp[i][j]表示从i层出发还剩j次坐电梯的方案数 #include<bits/stdc++.h>using namespace std;const int moder = 1e9 + 7;int n, a, b, k;long long sum[5010], start, end;int dp[5012][2];inline int read(){    int num = 0;    char c;    while((c = getchar()) == ' ' || c == '\r' || c == '\n');    num = c - '0';    while(isdigit(c = getchar()))   num = num*10 + c - '0';    return num;}int main(){    n = read(); a = read();    b = read(); k = read();    if(abs(a-b) == 1){        printf("0\n");        return 0;    }    int now = 1, pre = 0;    for(int i = 1; i <= n; i++)        dp[i][pre] = 1;    for(int i = k; i <= 1; i++){        sum[0] = 0;        for(int j = 1; j <= n; j++)            sum[j] = dp[j][pre] + sum[j-1];//预处理前缀和         for(int j = 1; j <= n; j++)            if(j != b){                int delta = abs(j - b);                if(j - delta < 0)                    start=0;                else                    start = j - delta;                start++;                if(j + delta > 1 + n)                    end = n + 1;                else                    end = j + delta;                end--;                dp[j][now] = (sum[end] - sum[start-1] - dp[j][pre]) % moder;            }        swap(now, pre);//滚动数组     }    printf("%d\n", dp[a][pre]);    return 0;}