38. Count and Say*
https://leetcode.com/problems/count-and-say/
题目描述
The count-and-say sequence is the sequence of integers with the first five terms as following:
1. 1
2. 11
3. 21
4. 1211
5. 111221
1
is read off as "one 1"
or 11
.
11
is read off as "two 1s"
or 21
.
21
is read off as "one 2, then one 1"
or 1211
.
Given an integer n
where 1 ≤ n ≤ 30
, generate the term of the count-and-say sequence. You can do so recursively, in other words from the previous member read off the digits, counting the number of digits in groups of the same digit.
Note: Each term of the sequence of integers will be represented as a string.
Example 1:
Input: 1
Output: "1"
Explanation: This is the base case.
Example 2:
Input: 4
Output: "1211"
Explanation: For n = 3 the term was "21" in which we have two groups "2" and "1", "2" can be read as "12" which means frequency = 1 and value = 2, the same way "1" is read as "11", so the answer is the concatenation of "12" and "11" which is "1211".
C++ 实现 1
给出第 6 个结果就知道题目啥意思了:
1. 1
2. 11
3. 21
4. 1211
5. 111221
6. 312211
发现用双指针就可以搞定, 计算 S[i, ..., j]
中的相同字符多少个. 下面是使用递归的做法.
class Solution {
public:
string countAndSay(int n) {
if (n == 1) return "1";
auto prev = countAndSay(n - 1);
string res;
for (int i = 0; i < prev.size(); ++ i) {
int j = i;
while (j + 1 < prev.size() && prev[j] == prev[j + 1]) ++ j;
auto count = j - i + 1;
res += std::to_string(count) + prev[i];
i = j;
}
return res;
}
};
C++ 实现 2
下面是使用迭代的做法:
class Solution {
public:
string countAndSay(int n) {
string prev = "1";
while (--n) {
string res = "";
for (int i = 0; i < prev.size();) {
int j = i;
while (j + 1 < prev.size() && prev[j + 1] == prev[i])
++ j;
res += to_string(j - i + 1) + prev[i];
i = j + 1;
}
prev = res;
}
return prev;
}
};