Problem 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)

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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:

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

Example 2:

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Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:

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

Solution:

Create a vector to store the characters in each row.
Traverse the string sequentially, the end condition is as follows.
vertically down: 0<= idx< numRows
obliquely up: 1<= idx <= numRows-2

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class Solution {
public:
    string convert(string s, int numRows) {
        vector<string> res(numRows);
        int curRow= 0, i= 0;
        
        while(i < s.length()){
            for(int idx= 0; i< s.length() && idx< numRows; idx++)
                res[idx].push_back(s[i++]);
            for(int idx= numRows-2; i< s.length() && idx>= 1; idx--)
                res[idx].push_back(s[i++]);
        }
        
        string _res;
        for(auto n: res){
            _res.append(n);
        }
        return _res;
    }
};

time complexity: $O(n)$
space complexity: $O(n)$
reference:
https://goo.gl/Jegxjj