add level.cpp

Author: kamyu104

C++/connect.cpp

@@ -0,0 +1,30 @@
+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+/**
+ * Definition for binary tree with next pointer.
+ * struct TreeLinkNode {
+ *  int val;
+ *  TreeLinkNode *left, *right, *next;
+ *  TreeLinkNode(int x) : val(x), left(NULL), right(NULL), next(NULL) {}
+ * };
+ */
+class Solution {
+    public:
+        void connect(TreeLinkNode *root) {
+            for(TreeLinkNode *list, *first; root; root = first) { // enumerate each level of depth
+                list = first = NULL;
+                for(; root; root = root->next) { // enumerate nodes at the same level of depth
+                    if(!first) first = (root->left)? root->left:root->right;
+                    if(root->left) {
+                        if(list) list->next = root->left;
+                        list = root->left;
+                    }
+                    if(root->right) {
+                        if(list) list->next = root->right;
+                        list = root->right;
+                    }
+                }
+            }
+        }
+};