leetcode翻转字符串541

/**
 * <p>给定一个字符串 <code>s</code> 和一个整数 <code>k</code>，从字符串开头算起，每计数至 <code>2k</code> 个字符，就反转这 <code>2k</code> 字符中的前 <code>k</code> 个字符。</p>
 *
 * <ul>
 * <li>如果剩余字符少于 <code>k</code> 个，则将剩余字符全部反转。</li>
 * <li>如果剩余字符小于 <code>2k</code> 但大于或等于 <code>k</code> 个，则反转前 <code>k</code> 个字符，其余字符保持原样。</li>
 * </ul>
 *
 * <p>&nbsp;</p>
 *
 * <p><strong>示例 1：</strong></p>
 *
 * <pre>
 * <strong>输入：</strong>s = "abcdefg", k = 2
 * <strong>输出：</strong>"bacdfeg"
 * </pre>
 *
 * <p><strong>示例 2：</strong></p>
 *
 * <pre>
 * <strong>输入：</strong>s = "abcd", k = 2
 * <strong>输出：</strong>"bacd"
 * </pre>
 *
 * <p>&nbsp;</p>
 *
 * <p><strong>提示：</strong></p>
 *
 * <ul>
 * <li><code>1 &lt;= s.length &lt;= 10<sup>4</sup></code></li>
 * <li><code>s</code> 仅由小写英文组成</li>
 * <li><code>1 &lt;= k &lt;= 10<sup>4</sup></code></li>
 * </ul>
 * <div><div>Related Topics</div><div><li>双指针</li><li>字符串</li></div></div><br><div><li> 336</li><li> 0</li></div>
 */

//leetcode submit region begin(Prohibit modification and deletion)
class Solution {
    public static String reverseStr(String s, int k) {

        for (int i = 0; i < s.length(); i += (k * 2)) {
            if (i + k < s.length()) {
                s = reverse(s, i, i + k - 1);
            } else {
                s = reverse(s, i, s.length() - 1);
            }
        }
        return s;
    }
    //注意 swap之后要重新生成字符串
    static String reverse(String s, int start, int end) {
        char[] nums = s.toCharArray();
        char temp;
        while (start < end) {
            temp = nums[start];
            nums[start] = nums[end];
            nums[end] = temp;
            start++;
            end--;
        }
        return new String(nums);
    }
}
//leetcode submit region end(Prohibit modification and deletion)