URL化 LeetCode

```java
package com.stan.algo.leetcode;
public class URLReplaceSpaces {
    public static void main(String[] args) {
        System.out.println(replaceSpaces("Mr John Smith    ", 13));
        System.out.println(replaceSpaces("               ", 5));
    }
    /**
     * 输入："Mr John Smith    ", 13
     * 输出："Mr%20John%20Smith"
     * <p>
     * 输入："               ",
     * 输出："%20%20%20%20%20"
     *
     * @param S
     * @param length
     * @return
     */
    public static String replaceSpaces(String S, int length) {
        // 转为字符数组
        char[] chars = S.toCharArray();
        int count = 0;
        int i = 0;
        // 时间复杂度 O(n)
        for (; i < length; i++) {
            if (chars[i] != 32) {
                count++;
            }
        }
        char[] chars1;
        if (count == 0) {
            chars1 = new char[length * 3];
            for (int j = 0; j < length * 3; j++) {
                if (j % 3 == 0) {
                    chars1[j] = '%';
                } else if (j % 3 == 1) {
                    chars1[j] = '2';
                } else {
                    chars1[j] = '0';
                }
            }
            return new String(chars1);
        }
        chars1 = new char[3 * (length) - 2 * count];
        int index = 0;
        for (int j = 0; j < length; j++) {
            if (chars[j] == 32) {
                chars1[index++] = '%';
                chars1[index++] = '2';
                chars1[index++] = '0';
            } else {
                chars1[index] = chars[j];
                index++;
            }
        }
        return new String(chars1);
    }
}