5. 剑指Offer系列——面试题05. 替换空格

题目：请实现一个函数，把字符串 s 中的每个空格替换成"%20"

示例 1：

输入：s = "We are happy."
输出："We%20are%20happy."

方法一：

• 直接使用java内部方法
• 用的StringBuffer，效率略低

public String replaceSpace(String s) {
    // 用的StringBuffer，效率略低
    return s.replaceAll(" ", "%20");
}

方法二：

• 遍历每个字符，逐个替换
• 时间复杂度：O(n)O(n)。遍历字符串 s 一遍
• 空间复杂度：O(n)O(n)。额外创建字符数组，长度为 s 的长度的 3 倍

public String replaceSpace(String s) {
    int length = s.length();
    char[] array = new char[length * 3];
    int size = 0;
    for (int i = 0; i < length; i++) {
        char c = s.charAt(i);
        if (c == ' ') {
            array[size++] = '%';
            array[size++] = '2';
            array[size++] = '0';
        } else {
            array[size++] = c;
        }
    }
    return new String(array, 0, size);
}

方法三：

• 遍历每个字符，逐个替换
• 使用StringBuilder

public String replaceSpace(String s) {
    StringBuilder stringBuilder = new StringBuilder();
    for (char c: s.toCharArray()) {
        if (c == ' ') {
            stringBuilder.append("%20");
        } else {
            stringBuilder.append(c);
        }
    }
    return stringBuilder.toString();
}

方法四：

• 用空格切分成字符串数组
• 用%20连接数组的每个元素

public String replaceSpace(String s) {
    StringBuilder stringBuilder = new StringBuilder();
    String[] array = s.split(" ");
    for (int i = 0; i < array.length; i++) {
        if (i != array.length - 1) {
            stringBuilder.append(array[i]).append("%20");
            continue;
        }
        stringBuilder.append(array[i]);
    }

    return stringBuilder.toString();
}

Test

@Test
public void testReplaceSpace() {
    Algorithm algorithm = new Algorithm();

    String s =  "We are happy.";
    Long start1 = System.nanoTime();
    System.out.println(algorithm.replaceSpace(s));
    Long end1 = System.nanoTime();
    System.out.println("time used: " + (end1 - start1));
    System.out.println("===============================================");

    Long start2 = System.nanoTime();
    System.out.println(algorithm.replaceSpace2(s));
    Long end2 = System.nanoTime();
    System.out.println("time used: " + (end2 - start2));
    System.out.println("===============================================");

    Long start3 = System.nanoTime();
    System.out.println(algorithm.replaceSpace3(s));
    Long end3 = System.nanoTime();
    System.out.println("time used: " + (end3 - start3));
    System.out.println("===============================================");

    Long start4 = System.nanoTime();
    System.out.println(algorithm.replaceSpace4(s));
    Long end4 = System.nanoTime();
    System.out.println("time used: " + (end4 - start4));
}

结果

We%20are%20happy.
time used: 95500
===============================================
We%20are%20happy.
time used: 20400
===============================================
We%20are%20happy.
time used: 24800
===============================================
We%20are%20happy.
time used: 349900