6 zigzag conversion of leetcode

1.Description

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);

2. TC

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"

3.Solution

public String convert(String s, int numRows) {
    
      if(numRows == 1)
          return s;
       
        List<StringBuilder> ans = new ArrayList<>();
        
        for (int i = 0; i <= Math.min(numRows, s.length()); i++) {
    
            ans.add(new StringBuilder());
        }
        
        boolean goingDown = false;
        
        int curRow = 0;
        
        for(char c : s.toCharArray()) {
    
            ans.get(curRow).append(c);
            if (curRow == 0 || curRow == numRows -1)
                goingDown = !goingDown;
            curRow += goingDown ? 1 : -1;
        }
        
       StringBuilder sb = new StringBuilder();
        for(StringBuilder chars: ans){
    
                sb.append(chars);
        }
        return sb.toString();
    }

Ideas ：
Then use one list Store the contents of each line , The content of each line is incoherent , So with StringBuilder To concatenate strings .
Then decide which line to go in according to the direction of going up or down again .
Finally, put all StringBuilder Spliced into a complete string .

public String convert(String s, int numRows) {
    
      if(numRows == 1)
          return s;
        StringBuilder sb = new StringBuilder();
        int n  = s.length();
        int df = 2*numRows -2;
        
        for (int i = 0; i < numRows; i++) {
    
            for (int j = 0; j + i < n; j+=df ) {
    
                sb.append(s.charAt(i + j));
                if (i != 0 && i != numRows - 1 && j -i +df < n) {
    
                    sb.append(s.charAt(j - i + df));
                }
            }
        }
        return sb.toString();
    }

Read line by line Z The content of the font , To calculate the difference between two points , Find the law .

P     I    N
A   L S  I G
Y A   H R
P     I

Pictured above is an example , Three characters in the first line P, I, N, The subscripts in the original string are 0, 6, 12, The difference is 6, This value is related to the number of rows ,3 Line difference is 4, The rule is 2* Row number -2;
For the number of rows in the middle , Namely j-i+df