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Maximum Depth of Binary Tree

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.

树的深度是从根节点开始(其深度为1)自顶向下逐层累加的,而高度是从叶节点开始(其高度为1)自底向上逐层累加的。

class Solution {
private:
    int depth = 0;
    void maxDepthUtil(TreeNode *root , int d)
    {
        if(root == NULL)
            return;
        if(d > depth)
            depth = d;
        maxDepthUtil( root->left, d+1);
        maxDepthUtil( root->right, d+1);
    }
public:
    int maxDepth(TreeNode *root) {
        maxDepthUtil( root,1 );
        return depth;
    }
};

//另一种写法
class Solution {
public:
    int maxDepth(TreeNode *root) {
        if(root == NULL)
            return 0;
        return max( maxDepth(root->left),maxDepth(root->right) )+1;
    }
};