Update Merge Two Binary Trees.md

Author: cheatsheet1999

JS-Algo/Merge Two Binary Trees.md

@@ -39,3 +39,30 @@ var mergeTrees = function(t1, t2) {
     return t1
 };
 ```
+
+
+Extra: Stack solution
+
+```JS
+const mergeTrees = (t1, t2) => {
+    if (!t1 || !t2)
+        return t1 || t2;
+    const stack = Array();
+    stack.push([t1, t2]);
+    while (stack.length > 0) {
+        const pair = stack.pop();
+        if (!pair[0] || !pair[1])
+            continue;
+        pair[0].val += pair[1].val;
+        if (!pair[0].left)
+            pair[0].left = pair[1].left;
+        else
+            stack.push([pair[0].left, pair[1].left]);
+        if (!pair[0].right)
+            pair[0].right = pair[1].right;
+        else
+            stack.push([pair[0].right, pair[1].right]);
+    }
+    return t1;
+};
+```