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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"

Constraints:

  • 1 <= s.length <= 1000
  • s consists of English letters (lower-case and upper-case), ',' and '.'.
  • 1 <= numRows <= 1000

Solution:

class Solution {
    public String convert(String s, int numRows) {
        if (numRows <= 1 || numRows >= s.length()){
            return s;
        }

        List<StringBuilder> rows = new ArrayList<>(numRows);
        for(int i = 0; i < numRows; i++){
            rows.add(new StringBuilder());
        }

        int cur = 0;
        int dir = -1;
        for (char ch : s.toCharArray()){
            rows.get(cur).append(ch);

            if (cur == 0 || cur == numRows - 1){
                dir = - dir;
            }
            cur = cur + dir;
        }

        StringBuilder result = new StringBuilder();
        for (StringBuilder row : rows){
            result.append(row);
        }

        return result.toString();
    }
}

// TC: O(n)
// SC: O(n)