洛谷P1970 花匠(dp)

题意

题目链接

sol

直接用\(f[i][0/1]\)表示到第\(i\)个位置，该位置是以上升结尾还是以下降结尾

转移的时候只需枚举前一个即可

#include<cstdio>
#include<algorithm>
using namespace std;
const int maxn = 1e6 + 10;
inline int read() {
    char c = getchar(); int x = 0, f = 1;
    while(c < '0' || c > '9') {if(c == '-') f = -1; c = getchar();}
    while(c >= '0' && c <= '9') x = x * 10 + c - '0', c = getchar();
    return x * f;
}
int n, a[maxn], f[maxn][2];
int main() {
    n = read();
    for(int i = 1; i <= n; i++) {
        a[i] = read();
        if(i == 1) f[i][0] = f[i][1] = 1;
        else {
            f[i][0] = f[i - 1][0];
            f[i][1] = f[i - 1][1];
            if(a[i] > a[i - 1]) f[i][1] = max(f[i][1], f[i - 1][0] + 1);
            if(a[i] < a[i - 1]) f[i][0] = max(f[i][0], f[i - 1][1] + 1);
        }
    }
    printf("%d", max(f[n][1], f[n][0]));
    return 0;
}
/*
4 
2 3 4 5
*/