public String reverseStr(String s, int k) {
		char[] ch = s.toCharArray();
		char tmp;
		for(int i = 0;i < ch.length; i+=2*k) {
			int start = i;
			//这里是判断尾数够不够k个来取决end指针的位置
			int end = Math.min(ch.length - 1, start + k -1);
			//用异或运算反转 
			while(start < end) {
				tmp = ch[start];
				ch[start] = ch[end];
				ch[end] = tmp;
				start++;
				end--;
			}
		}
		return new String(ch);
	}