思路：先確定root，在遞迴獲取root.left和root.right
# definition for a binary tree node.
# class treenode(object):
# def __init__(self, x):
# self.val = x
# self.left = none
# self.right = none
class
solution
(object):
defgeneratetrees
(self, n):
""" :type n: int
:rtype: list[treenode]
"""arrays = [i for i in range(1,n+1)]
return self.dp(arrays)
defdp(self,arrays):
if len(arrays) == 1:
return [treenode(arrays[0])]
ret =
for k in range(len(arrays)):
num = arrays[k]
l = arrays[0:k]
r = arrays[k+1:]
left = self.dp(l)
right = self.dp(r)
if left == :
for j in range(len(right)):
node = treenode(num)
node.right = right[j]
if right == :
for i in range(len(left)):
node = treenode(num)
node.left = left[i]
for i in range(len(left)):
for j in range(len(right)):
node = treenode(num)
node.left = left[i]
node.right = right[j]
return ret
二叉搜尋樹的性質是中序遍歷結果為有序陣列，利用這個性質先獲取原樹中序遍歷的結果，然後尋找最高波峰和最低低谷，再用dfs來修正原來的樹即可
# definition for a binary tree node.
# class treenode(object):
# def __init__(self, x):
# self.val = x
# self.left = none
# self.right = none
import sys
class
solution
(object):
defrecovertree
(self, root):
""" :type root: treenode
:rtype: void do not return anything, modify root in-place instead.
"""stack = [root]
arrays = [-sys.maxint]
while(stack):
item = stack.pop()
if item.left == none
and item.right == none:
else:
if item.right:
if item.left:
val1,val2 = -sys.maxint,sys.maxint
for i in range(1,len(arrays)-1):
if arrays[i]>arrays[i-1] and arrays[i]>arrays[i+1] and arrays[i]>val1:
val1 = arrays[i]
if arrays[i]1] and arrays[i]1] and arrays[i]def
dfs(root,val1,val2):
if root.val == val1:
root.val = val2
elif root.val == val2:
root.val = val1
if root.left:
dfs(root.left,val1,val2)
if root.right:
dfs(root.right,val1,val2)
dfs(root,val1,val2)
設定最大最小邊界進行遞迴判斷
# definition for a binary tree node.
# class treenode(object):
# def __init__(self, x):
# self.val = x
# self.left = none
# self.right = none
import sys
class
solution
(object):
defisvalidbst
(self, root):
""" :type root: treenode
:rtype: bool
"""min_ = -sys.maxint
max_ = sys.maxint
return self.checkbst(root,min_,max_)
defcheckbst
(self,root,min_,max_):
if root is
none: return
true
if root.val <= min_ or root.val >= max_:
return
false
return self.checkbst(root.left,min_,root.val) and self.checkbst(root.right,root.val,max_)
計算節點的值為該節點的值+所有子孫節點值的和
# definition for a binary tree node.
# class treenode(object):
# def __init__(self, x):
# self.val = x
# self.left = none
# self.right = none
class
solution
(object):
defprunetree
(self, root):
""" :type root: treenode
:rtype: treenode
"""if self.nodevalues(root) == 0: return
none
root.left = self.prunetree(root.left)
root.right = self.prunetree(root.right)
return root
defnodevalues
(self,root):
if root is
none: return
0return root.val+self.nodevalues(root.left)+self.nodevalues(root.right)
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