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

Example 3:

Input: s = "A", numRows = 1
Output: "A"

JAVA

public class ZigZag_Conversion {

    public class Solution {
        public String convert(String s, int nRows) {
            if (s == null || s.length() <= nRows || nRows == 1) {
                return s;
            }
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < nRows; i++) {
                if (i == 0 || i == nRows - 1) {
                    int index = i;
                    while (index < s.length()) {
                        sb.append(s.charAt(index));
                        index += 2 * (nRows - 1);
                    }
                } else {
                    int index = i;
                    while (index < s.length()) {
                        sb.append(s.charAt(index));
                        if (index + 2 * nRows - 2 * i - 2 < s.length()) {
                            sb.append(s.charAt(index + 2 * nRows - 2 * i - 2));
                        }
                        index += 2 * (nRows - 1);
                    }
                }
            }
            return sb.toString();
        }
    }
}

############

class Solution {
    public String convert(String s, int numRows) {
        if (numRows == 1) {
            return s;
        }
        StringBuilder ans = new StringBuilder();
        int group = 2 * numRows - 2;
        for (int i = 1; i <= numRows; i++) {
            int interval = i == numRows ? group : 2 * numRows - 2 * i;
            int idx = i - 1;
            while (idx < s.length()) {
                ans.append(s.charAt(idx));
                idx += interval;
                interval = group - interval;
                if (interval == 0) {
                    interval = group;
                }
            }
        }
        return ans.toString();
    }
}

C++

class Solution {

public:
    string convert(string s, int numRows) {
        if (numRows == 1) return s;
        int N = s.size(), d = (numRows - 1) * 2;
        string ans;
        for (int i = 0; i < numRows; ++i) {
            int w = 2 * i;
            for (int j = i; j < N;) {
                ans += s[j];
                w = d - w;
                if (!w) w = d;
                j += w;
            }
        }
        return ans;
    }
};

PYTHON

class Solution(object):


  def convert(self, s, numRows):
    """
    :type s: str
    :type numRows: int
    :rtype: str
    """
    if numRows <= 1:
      return s
    n = len(s)
    ans = []
    step = 2 * numRows - 2
    for i in range(numRows):
      one = i
      two = -i
      while one < n or two < n:
        if 0 <= two < n and one != two and i != numRows - 1:
          ans.append(s[two])
        if one < n:
          ans.append(s[one])
        one += step
        two += step
    return "".join(ans)

JAVASCRIPT

    





    var convert = function (s, numRows) {
    if (numRows == 1) return s;
    let arr = new Array(numRows);
    for (let i = 0; i < numRows; i++) arr[i] = [];
    let mi = 0,
        isDown = true;
    for (const c of s) {
        arr[mi].push(c);

        if (mi >= numRows - 1) isDown = false;
        else if (mi <= 0) isDown = true;

        if (isDown) mi++;
        else mi--;
    }
    let ans = [];
    for (let item of arr) {
        ans = ans.concat(item);
    }
    return ans.join('');
};