Space Replacement
Question
- lintcode: (212) Space Replacement
Write a method to replace all spaces in a string with %20.
The string is given in a characters array, you can assume it has enough space
for replacement and you are given the true length of the string.
Example
Given "Mr John Smith", length = 13.
The string after replacement should be "Mr%20John%20Smith".
Note
If you are using Java or Python,please use characters array instead of string.
Challenge
Do it in-place.
題解
根據題意,給定的輸入陣列長度足夠長,將空格替換為%20
後也不會overflow。通常的思維為從前向後遍歷,遇到空格即將%20
插入到新陣列中,這種方法在生成新陣列時很直觀,但要求原地替換時就不方便了,這時可聯想到插入排序的做法——從後往前遍歷,空格處標記下就好了。由於不知道新陣列的長度,故首先需要遍歷一次原陣列,字符串類題中常用方法。
需要注意的是這個題並未說明多個空格如何處理,如果多個連續空格也當做一個空格時稍有不同。
Java
public class Solution {
/**
* @param string: An array of Char
* @param length: The true length of the string
* @return: The true length of new string
*/
public int replaceBlank(char[] string, int length) {
if (string == null) return 0;
int space = 0;
for (char c : string) {
if (c == ' ') space++;
}
int r = length + 2 * space - 1;
for (int i = length - 1; i >= 0; i--) {
if (string[i] != ' ') {
string[r] = string[i];
r--;
} else {
string[r--] = '0';
string[r--] = '2';
string[r--] = '%';
}
}
return length + 2 * space;
}
}
class Solution {
public:
/**
* @param string: An array of Char
* @param length: The true length of the string
* @return: The true length of new string
*/
int replaceBlank(char string[], int length) {
int space = 0;
for (int i = 0; i < length; i++) {
if (string[i] == ' ') space++;
}
int r = length + 2 * space - 1;
for (int i = length - 1; i >= 0; i--) {
if (string[i] != ' ') {
string[r] = string[i];
r--;
} else {
string[r--] = '0';
string[r--] = '2';
string[r--] = '%';
}
}
return length + 2 * space;
}
};
源碼分析
先遍歷一遍求得空格數,得到『新陣列』的實際長度,從後往前遍歷。
複雜度分析
遍歷兩次原陣列,時間複雜度近似為 , 使用了r
作為標記,空間複雜度 .