6. ZigZag Conversion

The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

P   A   H   N
A P L S I I G
Y   I   R

And then read line by line: "PAHNAPLSIIGYIR"

Write the code that will take a string and make this conversion given a number of rows:

string convert(string s, int numRows);

Example 1:

Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"

Example 2:

Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:

P     I    N
A   L S  I G
Y A   H R
P     I

class Solution {
public String convert(String s, int nRows) {  
        if(s == null || s.length()==0 || nRows <=0)  
            return "";  
        if(nRows == 1)  
            return s;
            
        StringBuilder res = new StringBuilder();  
        int size = 2*nRows-2;  
        for(int i=0;i<nRows;i++){  
            for(int j=i;j<s.length();j+=size){  
                res.append(s.charAt(j));  
                if(i != 0 && i != nRows - 1){//except the first row and the last row
                    int temp = j+size-2*i;
                    if(temp<s.length())
                        res.append(s.charAt(temp));
                }
            }                  
        }  
        return res.toString();  
    }
}

2. simulation，numRows个stringbuffer，最后拼起来

class Solution {
    public String convert(String s, int numRows) {
        StringBuilder[] sb = new StringBuilder[numRows];
        for (int i = 0; i < sb.length; i++) sb[i] = new StringBuilder();
        int le = s.length(), ind = 0;
        char[] ch = s.toCharArray();
        
        while(ind < le){
            for(int i = 0; i < numRows && ind < le; i++){
                sb[i].append(ch[ind++]);//纵向一列
            }
            for(int i = numRows - 2; i > 0 && ind < le; i--){
                sb[i].append(ch[ind++]);//处理非首尾行的情况
            }
        }
        StringBuilder st = new StringBuilder();
        for(int i = 0; i < numRows; i++) st.append(sb[i]);
        return st.toString();
    }
}