541. Reverse String II

Given a string and an integer k, you need to reverse the first k characters for every 2k characters counting from the start of the string. If there are
less than k characters left, reverse all of them. If there are less than 2k but greater than or equal to k characters, then reverse the first k characters and left the other as original.

Example:

Input: s = "abcdefg", k = 2
Output: "bacdfeg"

Restrictions:
The string consists of lower English letters only.
Length of the given string and k will in the range [1, 10000]

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public class Solution {
public String reverseStr(String s, int k) {
StringBuilder re = new StringBuilder();
char[] data = new char[k];
int n = 0;
boolean flag = true;
for (int i = 0; i < s.length(); ++i) {
if (n < k) {
data
= s.charAt(i);
++n;
continue;
}
i -= 1;
if (flag) {
while (--n >= 0)
re.append(data
);
n = 0;
flag = false;
} else {
for (int j = 0; j < n; ++j)
re.append(data[j]);
flag = true;
n = 0;
}
}
if (flag) {
while (--n >= 0)
re.append(data
);
n = 0;
flag = false;
} else {
for (int j = 0; j < n; ++j)
re.append(data[j]);
flag = true;
n = 0;
}
return re.toString();
}
}