# 536. Construct Binary Tree from String

## 刷题内容

• https://leetcode.com/problems/construct-binary-tree-from-string

You need to construct a binary tree from a string consisting of parenthesis and integers.

The whole input represents a binary tree. It contains an integer followed by zero, one or two pairs of parenthesis. The integer represents the root's value and a pair of parenthesis contains a child binary tree with the same structure.

You always start to construct the left child node of the parent first if it exists.

Example:
Input: "4(2(3)(1))(6(5))"
Output: return the tree root node representing the following tree:

4
/   \
2     6
/ \   /
3   1 5
Note:
There will only be '(', ')', '-' and '0' ~ '9' in the input string.
An empty tree is represented by "" instead of "()".


## 解题方案

class Solution(object):
def str2tree(self, s):
"""
:type s: str
:rtype: TreeNode
"""
if not s or len(s) == 0:
return None
if '(' not in s:
return TreeNode(int(s))

def paren_pair_idx(s):
paren_count = 0
for i in range(len(s)):
if s[i] == '(':
paren_count += 1
elif s[i] == ')':
paren_count -= 1
if paren_count == 0 and i > s.find('('):
return (s.find('('), i)

root = TreeNode(int(s[:s.find('(')]))
(paren_left, paren_right) = paren_pair_idx(s)
root.left = self.str2tree(s[paren_left+1: paren_right])
if paren_right < len(s) - 1:
root.right = self.str2tree(s[paren_right+2:-1])
else:
root.right = None
return root


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