104. Maximum Depth of Binary Tree

递归形式

    int maxDepth(TreeNode* root) {        if(!root) return 0;        return 1+max(maxDepth(root->left),maxDepth(root->right));    }

使用BFS

    int maxDepth(TreeNode* root) {        if(!root) return 0;        int res=0;        queue<TreeNode*> q;        q.push(root);        while(!q.empty()){            res++;            int n=q.size();            for(int i=0;i<n;i++){                TreeNode* t=q.front();q.pop();                if(t->left) q.push(t->left);                if(t->right) q.push(t->right);            }        }        return res;    }