code: public StrBuilder insert(int index, char c) {
  		moveDataAfterIndex(index, 1);
  		value[index] = c;
  		this.position = Math.max(this.position, index) + 1;
  		return this;
  	}
docs: 插入指定字符
  
  @param index 位置
  @param c 字符
  @return this