another recursive solution

Author: ignacio-chiazzo

114 Flatten Binary Tree to Linked List.js

@@ -16,21 +16,44 @@
 var flatten = function(root) {
     var stack = [];
     var p = root;
-    
+
     while(p !== null || stack.length !== 0){
         if(p.right  !== null){
             stack.push(p.right);
         }
-        
+
         if(p.left !== null){ // [!!!]point of confusing, if null then pop stack
             p.right = p.left;
             p.left = null;
         } else if(stack.length !== 0){
             var node = stack.pop();
             p.right = node;
         }
-        
+
         p = p.right;
     }
 };
 
+// Recursive solution
+
+var flatten = function(root) {
+    if(root === null || (root.left === null && root.right === null)) {
+        return;
+    }
+
+    var rootLeft = root.left;
+    var rootRight = root.right;
+    root.left = null;
+    root.right = null;
+
+    flatten(rootLeft);
+    flatten(rootRight);
+
+    root.right = rootLeft;
+
+    var aux = root;
+    while(aux !== null && aux.right !== null) {
+        aux = aux.right;
+    }
+    aux.right = rootRight;
+};