0006 - Zigzag Conversion (Medium)

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

string convert(string s, int numRows);

Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"

Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:
P     I    N
A   L S  I G
Y A   H R
P     I

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

// simulate and add each character to the corresponding row
// go down -> reach bottom -> go up -> reach top -> go down ...
class Solution {
public:
    string convert(string s, int n) {
        // edge case
        if (n == 1) return s;
        vector<string> rows(n);
        // j is the index to track which rows a character should be added to
        // d is the direction: -1 means go up, 1 means go down
        int j = 0, d = 1;
        for (int i = 0; i < s.size(); i++) {
            // add the current character to corresponding row
            rows[j] += s[i];
            // if it reaches to the last row, we need to go up
            if(j == n - 1) d = -1;
            // if it reaches to the first row, we need to go down
            else if(j == 0) d = 1;
            // move j pointer
            j += d;
        }
        // rows would look like below in the first example
        // ['PAHN', 'APLSIIG', 'YIR']
        // we use `accumulate` to build the final answer (in C++ 20, it takes O(n) only)
        return accumulate(rows.begin(), rows.end(), string{});
    }
};

# simulate and add each character to the corresponding row
# go down -> reach bottom -> go up -> reach top -> go down ...
class Solution:
    def convert(self, s: str, n: int) -> str:
        # edge case
        if n == 1: return s
        rows = ['' for _ in range(n)]
        # j is the index to track which rows a character should be added to
        # d is the direction: -1 means go up, 1 means go down
        j, d = 0, 1
        for i in range(len(s)):
            # add the current character to corresponding row
            rows[j] += s[i]
            # if it reaches to the last row, we need to go up
            if j == n - 1: d = -1
            # if it reaches to the first row, we need to go down
            elif j == 0: d = 1
            # move j pointer
            j += d;
        # rows would look like below in the first example
        # ['PAHN', 'APLSIIG', 'YIR']
        # we use join to build the final answer
        return ''.join(rows)