Leetcode: Count and Say

题目:

The count-and-say sequence is the sequence of integers beginning as follows:
1, 11, 21, 1211, 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, generate the nth sequence.

Note: The sequence of integers will be represented as a string.

题目描述不清, WA 了一次

Input:6 Output:"21112211" Expected:"312211"

思路:

1. 计数, 前进

代码:

class Solution {
public:
    string countAndSay(int n) {
        string prestr = "1", curstr;
		if(n == 1)
			return prestr;

		for(int i = 1; i < n; i ++) {
			curstr = generateNext(prestr);
			prestr = curstr;
		}
		return curstr;
    }
	string generateNext(const string &prestr) {
		string str;
		for(int i = 0; i < prestr.size(); i ++) {
			int n = countSame(prestr, i);
			str.push_back(n+'0');
			str.push_back(prestr[i]);
			i += n-1;
		}
		return str;
	}
	int countSame(const string &prestr, int i) {
		if(i >= prestr.size()) return 0;

		int cnt = 1, prechar = prestr[i];
		i++;
		while(i < prestr.size()) {
			if(prestr[i] == prechar) {
				cnt++;
				i++;
			}else{
				return cnt;
			}
		}
		return cnt;
	}
};