表达式为a*b-c的二叉树的代码

class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None

def buildTree(s: str) -> TreeNode:
    if not s:
        return None
    stack = []
    i = 0
    while i < len(s):
        if s[i].isdigit() or s[i] == '-':
            start = i
            while i < len(s) - 1 and (s[i + 1].isdigit() or s[i + 1] == '-'):
                i += 1
            num = int(s[start:i + 1])
            node = TreeNode(num)
            if stack:
                if not stack[-1].left:
                    stack[-1].left = node
                else:
                    stack[-1].right = node
            stack.append(node)
        elif s[i] == '*' or s[i] == '/':
            stack[-1].val = s[i]
        elif s[i] == '+':
            if stack[-1].val == '*' or stack[-1].val == '/':
                node = stack.pop()
                if not stack[-1].left:
                    stack[-1].left = node
                else:
                    stack[-1].right = node
            stack[-1].val = s[i]
        elif s[i] == '-':
            if i == 0 or s[i - 1] in ['+', '-', '*', '/']:
                start = i
                while i < len(s) - 1 and (s[i + 1].isdigit() or s[i + 1] == '-'):
                    i += 1
                num = int(s[start:i + 1])
                node = TreeNode(num)
                if stack:
                    if not stack[-1].left:
                        stack[-1].left = node
                    else:
                        stack[-1].right = node
                stack.append(node)
            else:
                if stack[-1].val == '*' or stack[-1].val == '/':
                    node = stack.pop()
                    if not stack[-1].left:
                        stack[-1].left = node
                    else:
                        stack[-1].right = node
                stack[-1].val = s[i]
        i += 1
    return stack[0]

使用示例：

s = "a*b-c"
root = buildTree(s)

该代码实现了将字符串表达式转换为二叉树的过程，其中每个节点的值都是一个运算符或者数字。可以通过遍历该二叉树的方式求得表达式的值。