# 651. 4 Keys Keyboard

Imagine you have a special keyboard with the following keys:

`Key 1: (A)`: Print one ‘A’ on screen.

`Key 2: (Ctrl-A)`: Select the whole screen.

`Key 3: (Ctrl-C)`: Copy selection to buffer.

`Key 4: (Ctrl-V)`: Print buffer on screen appending it after what has already been printed.

Now, you can only press the keyboard for N times (with the above four keys), find out the maximum numbers of ‘A’ you can print on screen.

Example 1:

``````Input: N = 3
Output: 3
Explanation:
We can at most get 3 A's on screen by pressing following key sequence:
A, A, A
``````

Example 2:

``````Input: N = 7
Output: 9
Explanation:
We can at most get 9 A's on screen by pressing following key sequence:
A, A, A, Ctrl A, Ctrl C, Ctrl V, Ctrl V
``````

Note:

1. 1 <= N <= 50
2. Answers will be in the range of 32-bit signed integer.

``````class Solution {
public:
int maxA(int N) {
int res = N;
for (int i = 1; i < N - 2; ++i) {
res = max(res, maxA(i) * (N - 1 - i));
}
return res;
}
};
``````

``````class Solution {
public:
int maxA(int N) {
vector<int> dp(N + 1, 0);
for (int i = 0; i <= N; ++i) {
dp[i] = i;
for (int j = 1; j < i - 2; ++j) {
dp[i] = max(dp[i], dp[j] * (i - j - 1));
}
}
return dp[N];
}
};
``````

2 Keys Keyboard

https://discuss.leetcode.com/topic/97764/o-1-time-o-1-space-c-solution-possibly-shortest-and-fastest

https://discuss.leetcode.com/topic/97628/java-4-lines-recursion-with-step-by-step-explanation-to-derive-dp

LeetCode All in One 题目讲解汇总(持续更新中…)

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