513. 找树左下角的值

方法一：BFS模板的应用。

总结的DFS、BFS模板参考：https://www.cnblogs.com/panweiwei/p/13065661.html

class Solution(object):
    # BFS
    def findBottomLeftValue(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return
        res = []
        stack = [root]
        while stack:
            sizestack = len(stack)
            temp = []
            for i in range(sizestack):
                node = stack.pop(0)
                temp.append(node.val)
                if node.left:
                    stack.append(node.left)
                if node.right:
                    stack.append(node.right)
            res.append(temp)
        return res[-1][0]

BFS方法的改进：层序时先右后左，直接返回最后一个节点的值。

class Solution(object):
    # BFS改进
    def findBottomLeftValue3(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return
        stack = [root]
        while stack:
            node = stack.pop(0)
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
        return node.val

方法二：DFS模板的应用。

class Solution(object):
    # DFS
    def findBottomLeftValue2(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """

        def dfs(node, curdepth):
            if not node:
                return
            if curdepth > self.maxdepth:
                self.res = node.val
                self.maxdepth = curdepth
            dfs(node.left, curdepth + 1)
            dfs(node.right, curdepth + 1)

        self.maxdepth, self.res = 0, root.val
        dfs(root, 0)
        return self.res