LeetCode——Count and Say

Count and Say

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.

"Count and Say problem" Write a code to do following: 
n String to print 
0 1 
1 1 1 
2 2 1 
3 1 2 1 1 
... 
Base case: n = 0 print "1" 
for n = 1, look at previous string and write number of times a digit is seen and the digit itself. In this case, digit 1 is seen 1 time in a row... so print "1 1" 
for n = 2, digit 1 is seen two times in a row, so print "2 1" 
for n = 3, digit 2 is seen 1 time and then digit 1 is seen 1 so print "1 2 1 1" 
for n = 4 you will print "1 1 1 2 2 1" 

Consider the numbers as integers for simplicity. e.g. if previous string is "10 1" then the next will be "1 10 1 1" and the next one will be "1 1 1 10 2 1"

- Cartman on October 19, 2010 Report Duplicate | Flag 

Java代码：

public class Solution {    public String countAndSay(int n) {         if (n < 1) return null;            String str = "1";      StringBuilder sb = new StringBuilder();      for(int j = 2; j <= n; j++) {          int count = 1;          for (int i = 1; i < str.length(); i++) {                   if (str.charAt(i) == str.charAt(i - 1)) {                  count++;              } else {                  sb.append(count);                  sb.append(str.charAt(i-1));                  count = 1;              }          }          sb.append(count);          sb.append(str.charAt(str.length() - 1));          str = sb.toString();              sb.setLength(0);      }      return str;      }}