替换空格

剑指offer第四题，将空格替换为%20，一开始的实现

package com.zjy.sword2offer;

public class SpaceReplace {
	
	public static String replace(String a){
		if(a==null||a.length()==0)
			return null;
		
		int count = 0;
		int originalLen = a.length();
		
		for(int i=0;i<originalLen;i++)
		{
			if(a.charAt(i)==' ')
				count++;
		}
		
		int newLen = originalLen + count*2;
		int pOriginal = originalLen-1;
		int pNew = newLen-1;
		StringBuffer sb = new StringBuffer(newLen);
		
		while(pOriginal>=0 && pNew>pOriginal)
		{
			if(a.charAt(pOriginal)!=' ')
			{
				sb.insert(pNew--, a.charAt(pOriginal));
			}
			else
			{
				sb.insert(pNew-2, "%20");
				pNew -= 2;
				pNew--;
			}
			pOriginal--;
		}
		
		return sb.toString();
	}
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		String str = "I am happy.";
		String res = replace(str);
		System.out.println(res);
	}

}

一直报错如下

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException
	at java.lang.System.arraycopy(Native Method)
	at java.lang.AbstractStringBuilder.insert(Unknown Source)
	at java.lang.StringBuffer.insert(Unknown Source)
	at com.zjy.sword2offer.SpaceReplace.replace(SpaceReplace.java:27)
	at com.zjy.sword2offer.SpaceReplace.main(SpaceReplace.java:44)

猜想出错的原因在于StringBuffer初始化后内容是空的，不能直接从后边用setCharAt（）或者insert（）插入数据，否则报错，后来换了一个方式ok了，如下

package com.zjy.sword2offer;

public class SpaceReplace {
	
	public static String replace(String a){
		if(a==null||a.length()==0)
			return null;
		
		int count = 0;
		int originalLen = a.length();
		
		for(int i=0;i<originalLen;i++)
		{
			if(a.charAt(i)==' ')
				count++;
		}
		
		int newLen = originalLen + count*2;
		int pOriginal = originalLen-1;
		int pNew = newLen-1;
		StringBuffer sb = new StringBuffer(newLen);
		
		while(pOriginal>=0 && pNew>=0)
		{
			char ch = a.charAt(pOriginal);
			if(ch!=' ')
			{
				sb.append(ch);
			}
			else
			{
				sb.append("02%");
			}
			pOriginal--;
		}
		
		return sb.reverse().toString();
	}
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		String str = "I am happy.";
		String res = replace(str);
		System.out.println(res);
	}

}