Maximum Depth of Binary Tree leetcode

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

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非递归实现：

int maxDepth(TreeNode* root) {
    int maxDepth = ;
    if (root == nullptr)
        return maxDepth;
    stack<TreeNode*> sta;
    sta.push(root);
    TreeNode* lastRoot = root;
    while (!sta.empty())
    {
        root = sta.top();
        if (lastRoot != root->right)
        {
            if (lastRoot != root->left) {
                if (root->left != nullptr) {
                    sta.push(root->left);
                    continue;
                }
            }
            if (root->right != nullptr) {
                sta.push(root->right);
                continue;
            }
            maxDepth = sta.size() > maxDepth ? sta.size() : maxDepth;
        }
        lastRoot = root;
        sta.pop();
    }
    return maxDepth;
}

递归实现：

int maxDepth(TreeNode* root) {
    if (root == nullptr)
        return ;
    int leftDepth = maxDepth(root->left) + ;
    int rightDepth = maxDepth(root->right) + ;
    return leftDepth > rightDepth ? leftDepth : rightDepth;
}