https://leetcode.com/problems/zigzag-conversion/

zigzag-conversion.java

/**
Solution One
Two Pass
**/
class Solution {
    public String convert(String s, int numRows) {
        if (s == null || s.length() == 0 || numRows == 0) {
            return "";
        }
        if (numRows == 1) {
            return s;
        }
        StringBuilder[] rows = new StringBuilder[numRows];
        for (int i = 0; i < numRows; i++) {
            rows[i] = new StringBuilder();
        }
        
        int idx = 0;
        int len = s.length();
        while(idx < len) {
            for (int i = 0; i < numRows && idx < len; i++,idx++) {
                rows[i].append(s.charAt(idx));
            }
            for (int i = numRows - 2; i > 0 && idx < len; i--, idx++) {
                rows[i].append(s.charAt(idx));
            }
        }
        
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < numRows; i++) {
            sb.append(rows[i]);
        }
        
        return sb.toString();
    }
}


/**
Solution Two

One Pass
**/
class Solution {
    public String convert(String s, int numRows) {
        if (s == null || s.length() == 0 || numRows == 0) {
            return "";
        }
        if (numRows == 1) {
            return s;
        }
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < numRows; i++) {
            int prej = -1;
            int j = i;
            int[] steps = {2 * (numRows - i - 1), 2 * i};
            int stepIdx = 0;
            while(j < s.length()) {
                if (prej != j) {
                    sb.append(s.charAt(j));
                }
                prej = j;
                j += steps[stepIdx];
                stepIdx = 1 - stepIdx;
            }
        }
        
        return sb.toString();
    }
}