feat:add Solution.cpp for 0104.Maximum Depth of Binary Tree

Author: Stackingrule

solution/0100-0199/0104.Maximum Depth of Binary Tree/Solution.cpp

@@ -0,0 +1,9 @@
+class Solution {
+public:
+    int maxDepth(TreeNode* root) {
+        if (!root) return 0;
+        int l = maxDepth(root->left);
+        int r = maxDepth(root->right);
+        return max(l, r) + 1;
+    }
+};