Added solution for diameter of n ary tree.

C++/diameter-of-n-ary-tree.cpp

@@ -0,0 +1,42 @@
+/*
+// Definition for a Node.
+class Node {
+public:
+    int val;
+    vector<Node*> children;
+
+    Node() {}
+
+    Node(int _val) {
+        val = _val;
+    }
+
+    Node(int _val, vector<Node*> _children) {
+        val = _val;
+        children = _children;
+    }
+};
+*/
+
+class Solution {
+public:
+    int diameter(Node* root) {
+        int res = 0; // tc: O(n), sc: O(n)
+        dfs(root, res);
+        return res;
+    }
+    int dfs(Node* root, int& res) {
+        if(!root) return 0;
+        int first = 0, second = 0;
+        for(auto& child:root->children) {
+            int tmp = dfs(child, res);
+            if(tmp >= first) {
+                second = first;
+                first = tmp;
+            }
+            else if(tmp >= second) second = tmp;
+        }
+        res = max(res, first + second);
+        return max(first, second) + 1;
+    }
+};