ZigZag Conversion

Desicription

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:

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string convert(string text, int nRows);

convert("PAYPALISHIRING", 3) should return "PAHNAPLSIIGYIR".

Solution

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class Solution {
public:
string convert(string s, int numRows) {
if(numRows <= 1 || s.size() <= numRows)
return s;
string res;
int s_size = s.size();
for(int i = 0; i < numRows; i++){
if(i == 0 || i == numRows-1){
int index = i;
while(index < s_size){
res += s[index];
index += (numRows - 1) * 2;
}
}
else{
int index = i;
while(index < s_size){
res += s[index];
if(index + (numRows-i-1) * 2 < s_size)
res += s[index + (numRows-i-1) * 2];
index += (numRows - 1) * 2;
}
}
}
return res;
}
};