104 maximum depth of binary tree

分别递归计算左右树的最大高度，然后取最大值

int maxDepth(struct TreeNode* root) {

    if(root == NULL)
        return 0;
    int l, r;
    l = maxDepth(root->left);
    r = maxDepth(root->right);
    if(l > r)
        return l+1;
    else
        return r+1;

}