ZigZagConversion.java

package easy;

public class ZigZagConversion {

    public String convert(String s, int numRows) {
        StringBuilder[] sb = new StringBuilder[numRows];
        char[] c= s.toCharArray();
        int len = s.length();
        for(int i = 0; i < numRows; i++){
            sb[i] = new StringBuilder();//this is an important step to initialize it
        }
        int i = 0;
        while(i < len){
            for(int index = 0; index < numRows && i < len; index++){
                sb[index].append(c[i++]);// vertically down
            }
            
            for(int index = numRows - 2; index >= 1 && i < len; index--){/**Why it should start from numRows - 2? Think of the example when numRows = 3
            the starting point of obliquely going up is 1, which is numRows-2.*/
                sb[index].append(c[i++]);// obliquely up
            }
        }
        
        for(i = 1; i < numRows; i++){
            sb[0].append(sb[i]);
        }
        return sb[0].toString();
    }

}