Codeforces 789 C Functions again DP

Posted 2020-10-09 FriskyPuppy

　　题目链接： http://codeforces.com/problemset/problem/789/C

　　题目描述： 给定一个数列， 问从某项开始加一项减一项的最大值是多少

　　解题思路： 先把数列分成两种情况， 一种是第一个正第二个负第三个正......  一种是第一个负第二个正第三个负...... DP求一个连续子序列最大和就可以了

　　代码： 

#include <iostream>
#include <cstdio>
#include <map>
#include <iterator>
#include <string>
#include <algorithm>
#include <vector>
#include <cmath>
#include <cstring>
using namespace std;
typedef long long ll;

const ll maxn = 1e5+10;
ll a[maxn];
ll b[maxn];
ll c[maxn];
ll d[maxn];

ll solve( ll * p, ll len ) {
    ll ans = 0;
    ll cnt = 0;
    for( ll i = 1; i <= len; i++ ) {
        cnt += p[i];
        if( cnt < 0 ) cnt = 0;
        else {
            if( cnt > ans ) ans = cnt;
        }
    }
    return ans;
}
int main() {
    ll n;
    scanf( "%lld", &n );
    for( ll i = 1; i <= n; i++ ) {
        scanf( "%lld", a+i );
    }
    for( ll i = 1; i <= n-1; i++ ) {
        b[i] = abs(a[i+1]-a[i]);
    }
    n--;
    for( ll i = 1; i <= n; i++ ) {
        if( i & 1 ) {
            c[i] = -b[i];
            d[i] = b[i];
        }
        else {
            c[i] = b[i];
            d[i] = -b[i];
        }
    }
    printf( "%lld\n", max(solve(c,n), solve(d, n)));
    return 0;
}

View Code

　　思考： 训练自己的代码能力与将问题抽象出来的能力