ZigZagConversion.java

public class ZigZagConversion {
    public String convert(String s, int nRows) {
        // corner cases
        if(s == null) return null;
        if(s.equals("")) return s;
        if(nRows == 0) return null;
        if(nRows == 1) return s;
        
        StringBuilder[] builders = new StringBuilder[nRows];
        for(int i = 0; i < nRows; i++){
            builders[i] = new StringBuilder();
        }
        
        int nBetween = nRows - 2;
        int len = s.length();
        
        int count1 = 0;
        boolean inBody = true; // e.g., when processing P A Y 
        int count2 = 0;
        boolean inBetween = false; // e.g., when processing the single P in between
        
        for(int i = 0; i < len; i++){
            // pushing character to its corresponding builder
            if(inBody == true && count1 < nRows){
                builders[count1].append(s.charAt(i));
                count1++;
            }else if(inBetween == true && count2 < nBetween){
                builders[nRows - 2 - count2].append(s.charAt(i));
                count2++;
            }
            // state change between inBody and inBetween
            if(inBody == true && count1 == nRows){
                inBody = false;
                inBetween = true;
                count1 = 0;
            }else if(inBetween == true && count2 == nBetween){
                inBody = true;
                inBetween = false;
                count2 = 0;
            }
        }
        
        StringBuilder b = new StringBuilder();
        for(int i = 0; i < nRows; i++){
            b.append(builders[i].toString());
        }
        return b.toString();
    }
}