6. Zigzag Conversion

Medium
​
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

解題

func convert(s string, numRows int) string {
length := len(s)
answer := ""
​
if length == 0 || length<=numRows || numRows == 1 {
return s
}
​
for i := 0; i < numRows; i++ {
gap := (numRows-1)*2 - 2*i
gap2 := (numRows-1)*2 - gap
​
j := i
answer += string(s[j])
​
for {
if gap != 0 {
j += gap
if j >= length {
break
}
answer += string(s[j])
}
​
if gap2 != 0 {
j += gap2
if j >= length {
break
}
answer += string(s[j])
}
}
}
​
return answer
}