leetcode-Reverse Words in a String

Given an input string, reverse the string word by word.

For example,

Given s = "

the sky is blue

",

return "

blue is sky the

".

Update (2015-02-12):

For C programmers: Try to solve it in-place in O(1) space.

click to show clarification.

Clarification:

What constitutes a word?

A sequence of non-space characters constitutes a word.

Could the input string contain leading or trailing spaces?

Yes. However, your reversed string should not contain leading or trailing spaces.

How about multiple spaces between two words?

Reduce them to a single space in the reversed string.

题目解析：1、生成的单词里面没有空格！

　　　　　2、输入的String，包括头或尾空格？ 是的，反转的时候不能包含这些空格

3、两个单词之间的多个空格？ 只保留一个空格分析：两种方法：

代码1：

String[] parts = s.trim().split("\\s+");//去除前后的空格+去除里面的空格，得到一个没有空格的字符串

String out = "";

if (parts.length > 0) {   //从后往前输出，加入字符串

for (int i = parts.length - 1; i > 0; i--)

{ out += parts[i] + " "; }

out += parts[0]; }

return out;

代码2：通过反转整个字符串，去除不符合题目的空格；

再反转每一个单词；

package leetcode;

public class ReverseWordsinaString {
// reverses the part of an array and returns the input array for convenience
public static char[] reverse(char[] arr, int i, int j) {
while (i < j) {
char tmp = arr[i];
arr[i++] = arr[j];
arr[j--] = tmp;
}
return arr;
}
public static String reverseWords(String s) {
// reverse the whole string and convert to char array
char[] str = reverse(s.toCharArray(), 0, s.length()-1);
// System.out.println(str.toString());
int start = 0, end = 0; // start and end positions of a current word
for (int i = 0; i < str.length; i++) {
if (str[i] != ' ') { // if the current char is letter
str[end++] = str[i]; // just move this letter to the next free pos
} else if (i > 0 && str[i-1] != ' ') { // if the first space after word
reverse(str, start, end-1); // reverse the word
str[end++] = ' '; // and put the space after it
start = end; // move start position further for the next word
}
}
// System.out.println(s);
// System.out.println(s.length());
reverse(str, start, end-1);
//为什么要加上面那一步呢？？？貌似没有区别

// reverse the tail word if it's there
// here's an ugly return just because we need to return Java's String
// also as there could be spaces at the end of original string
// we need to consider redundant space we have put there before
// System.out.println(s);
// System.out.println(s.length());

return new String(str, 0, end > 0 && str[end-1] == ' ' ? end-1 : end);
}
public static void main(String[] args) {
System.out.println(reverseWords(" he ll o world!! "));
System.out.println(reverseWords(" he ll o world!! ").length());
}

}