如何删除树之间的空格

def remove_spaces_between_trees(input_str):
    result = ""
    i = 0
    while i < len(input_str):
        if input_str[i] == ' ':
            # 判断空格字符是否位于两个树节点之间
            if i > 0 and i < len(input_str) - 1 and input_str[i-1].isalpha() and input_str[i+1].isalpha():
                i += 1
                continue
        result += input_str[i]
        i += 1
    return result

# 示例用法
input_str = "tree1 tree2 tree3"
output_str = remove_spaces_between_trees(input_str)
print(output_str)  # 输出：tree1tree2tree3