Create 107-Binary-Tree-Level-Order-Traversal-II.java

Author: SharmaTushar1

java/107-Binary-Tree-Level-Order-Traversal-II.java

@@ -0,0 +1,31 @@
+//Same just reverse in the end.
+//Reversing in the end is better than using add(0, E) in case of arraylist as it's an O(1) operation.
+class Solution {
+    public List<List<Integer>> levelOrderBottom(TreeNode root) {
+        List<List<Integer>> ans = new ArrayList<>();
+        if (root == null) return ans;
+        Queue<TreeNode> q = new LinkedList<>();
+        q.offer(root);
+        while (!q.isEmpty()) {
+            List<Integer> level = new ArrayList<>();
+            int size = q.size();
+            for (int i = 0; i<size; i++) {
+                TreeNode cur = q.poll();
+                level.add(cur.val);
+                if (cur.left!=null) q.offer(cur.left);
+                if (cur.right!=null) q.offer(cur.right);
+            }
+            ans.add(level);
+        }
+        int i = 0 , j = ans.size()-1;
+        //reverse the list
+        while (i<j) {
+            List<Integer> cur = ans.get(i);
+            ans.set(i, ans.get(j));
+            ans.set(j, cur);
+            i++;
+            j--;
+        }
+        return ans;
+    }
+}