lintcode53 Reverse Words in a String -easy

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

For example,
Given s = "the sky is blue",
return "blue is sky the".

• 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

import java.util.Stack;
public class Solution {
    /*
     * @param s: A string
     * @return: A string
     */
    public String reverseWords(String s) {
        // write your code here

        if(s == null){
            return null;
        }

        if(s.length() == 0){
            return new String();
        }

        Stack<String> stack = new Stack<String>();
        int head = 0;
        int tail = 0;

        while(head < s.length()){
            while(head < s.length() && s.charAt(head) == ‘ ‘){
                head++;
            }
            tail = head;

            while(tail < s.length() && s.charAt(tail) != ‘ ‘){
                tail ++;
            }

            if(head >= s.length() || tail > s.length()){
                break;
            }

            stack.push(s.substring(head, tail));
            head = tail;
        }

        String reversed = new String();
        while (!stack.isEmpty()){
            reversed += stack.pop();
            reversed += " ";
        }
        if(reversed.length() > 0){
            reversed = reversed.substring(0, reversed.length() -1);
        }

        return reversed;
    }
}

1. str.substring(int, int); 而不是str.subString(int, int);
2. substring(int beginIndex,int endIndex)从指定的 beginIndex 处开始，直到索引 endIndex - 1 处的字符。因此，该子字符串的长度为 endIndex-beginIndex。 "smiles".substring(1, 5) returns "mile"
3. 在塞完所有单词到stack里后，最后一个个拿出来打印的时候，切记切记循环体for()里面不可以是int i = 0; i < stack.size(); i++!，因为你for{}里面的操作有在pop，那你等于size这个值就一直在变！可以

int wordCount = stack.size();
if(!stack.isEmpty()){
　　for(int i = 0; i < wordCount - 1; i++){
　　　　reversed+= stack.pop();
　　　　reversed+= " ";
　　}
　　reversed += stack.pop();
}

或者

while (!stack.isEmpty()){
            reversed += stack.pop();
            reversed += " ";
        }
        if(reversed.length() > 0){
            reversed = reversed.substring(0, reversed.length() -1);
        }}

4.注意关注输入处理，null, "", " "。在心里跑一下他们