21. 构造二叉树

find（）方法

在Python中，str.find(sub[, start[, end]]) 方法用于查找子字符串 sub 在字符串中首次出现的位置，返回其起始索引。如果未找到，返回 -1

class Tree:
    def __init__(self,val=None,left=None,right=None):
        self.left = left
        self.right = right
        self.val = val
def make_tree(pre_order,mid_order):
    if not pre_order or not mid_order:
        return None
    idx = mid_order.find(pre_order[0])
    left = make_tree(pre_order[1:idx+1],mid_order[:idx])
    right = make_tree(pre_order[idx+1:],mid_order[idx+1:])
    # head = Tree(val=head,left=left,right=right)
    return Tree(val=pre_order[0],left=left,right=right)
def behind_order(head):
    if head is None:
        return []
    left = behind_order(head.left)
    right = behind_order(head.right)
    val = head.val
    return left + right + [val]
while True:
    try:
        pre_order,mid_order = map(str,input().split(' '))
        # head = make_tree(pre_order,mid_order)
        # print(behind_order(make_tree(pre_order,mid_order)))
        print(''.join(behind_order(make_tree(pre_order,mid_order))))
    except:
        break