LeetCode-in-Java
541. Reverse String II
Easy
Given a string s and an integer k, reverse the first k characters for every 2k characters counting from the start of the string.
If there are fewer 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 1:
Input: s = “abcdefg”, k = 2
Output: “bacdfeg”
Example 2:
Input: s = “abcd”, k = 2
Output: “bacd”
Constraints:
1 <= s.length <= 104sconsists of only lowercase English letters.1 <= k <= 104
Solution
public class Solution {
public String reverseStr(String s, int k) {
StringBuilder res = new StringBuilder();
int p1 = 0;
int p2 = 2 * k - 1;
if (s.length() < k) {
res.append(reverse(s));
return res.toString();
}
while (p1 < s.length()) {
if (s.length() < p1 + k) {
res.append(reverse(s.substring(p1)));
} else {
res.append(reverse(s.substring(p1, p1 + k)));
if (s.length() < p2 + 1) {
res.append(s.substring(p1 + k));
} else {
res.append(s, p1 + k, p2 + 1);
}
}
p1 = p1 + 2 * k;
p2 = p2 + 2 * k;
}
return res.toString();
}
private String reverse(String s) {
StringBuilder sb = new StringBuilder();
sb.append(s);
sb.reverse();
return sb.toString();
}
}