DP Codeforces Round #303 (Div. 2) C. Woodcutters

题目传送门

 1 /*
 2     题意：每棵树给出坐标和高度，可以往左右倒，也可以不倒
 3     问最多能砍到多少棵树
 4     DP：dp[i][0/1/2] 表示到了第i棵树时，它倒左或右或不动能倒多少棵树
 5     分情况讨论，若符合就取最大值更新，线性dp，自己做出来了:)
 6 */
 7 #include <cstdio>
 8 #include <algorithm>
 9 #include <cstring>
10 #include <cmath>
11 #include <iostream>
12 using namespace std;
13
14 typedef long long LL;
15
16 const int MAXN = 1e5 + 10;
17 const int INF = 0x3f3f3f3f;
18 LL x[MAXN], h[MAXN];
19 LL dp[MAXN][3];        //0-l 1-no 2-r
20
21 int main(void)        //Codeforces Round #303 (Div. 2) C. Woodcutters
22 {
23     //freopen ("C.in", "r", stdin);
24
25     int n;
26     while (scanf ("%d", &n) == 1)
27     {
28         for (int i=1; i<=n; ++i)
29         {
30             scanf ("%I64d%I64d", &x[i], &h[i]);
31         }
32         memset (dp, 0, sizeof (dp));
33         dp[1][0] = dp[1][1] = 1;
34         if (x[1] + h[1] < x[2])    dp[1][2] = 1;
35         for (int i=2; i<=n; ++i)
36         {
37             LL tmp;
38             tmp = max (dp[i-1][0], dp[i-1][1]);
39             tmp = max (tmp, dp[i-1][2]);
40             dp[i][1] = tmp;
41             if (x[i] - h[i] > x[i-1])
42             {
43                 dp[i][0] = max (dp[i-1][0], dp[i-1][1]) + 1;
44             }
45             if (x[i] - x[i-1] > h[i-1] + h[i])
46             {
47                 dp[i][0] = max (dp[i][0], dp[i-1][2] + 1);
48             }
49             if (i < n && x[i] + h[i] < x[i+1])
50             {
51                 dp[i][2] = tmp + 1;
52             }
53             if (i == n)    dp[i][2] = tmp + 1;
54         }
55
56         LL ans;
57         ans = max (dp[n][0], dp[n][1]);
58         ans = max (ans, dp[n][2]);
59         printf ("%I64d\n", ans);
60     }
61
62     return 0;
63 }