[Sum number && build post order and inorder]

Sample.java

@@ -4,4 +4,63 @@
 // Any problem you faced while coding this :
 
 
-// Your code here along with comments explaining your approach
+// Your code here along with comments explaining your approach
+
+public class Sample {
+    // Time Complexity : O(n)
+    // Space Complexity : O(h)
+    // Did this code successfully run on Leetcode : yes
+    public int sumNumbers(TreeNode root) {
+        return helperSum(root, 0);
+    }
+    private int helperSum(TreeNode root, int currSum) {
+        if (root == null) {
+            return 0;
+        }
+        currSum = currSum * 10 + root.val;
+
+        if (root.left == null && root.right == null) {
+            return currSum;
+        }
+
+        return helperSum(root.left, currSum) + helperSum(root.right, currSum);
+    }
+
+
+
+
+
+
+
+    // Time Complexity : O(n)
+    // Space Complexity : O(h)
+    // Did this code successfully run on Leetcode : yes
+
+    HashMap<Integer, Integer> map = new HashMap<>();
+    int postorderIdx;
+    public TreeNode buildTree(int[] inorder, int[] postorder) {
+        postorderIdx = postorder.length - 1;
+        for (int i = 0; i < inorder.length; i++) {
+            map.put(inorder[i],i);
+        }
+
+        return helperTree(postorder, 0, postorderIdx);
+    }
+    public TreeNode helperTree(int[] postorder, int left, int right) {
+
+        if (left > right) {
+            return null;
+        }
+
+        int rootVal = postorder[postorderIdx];
+        postorderIdx--;
+        TreeNode root = new TreeNode(rootVal);
+
+        int inorderIdx = map.get(rootVal);
+
+        root.right = helperTree(postorder, inorderIdx + 1, right);
+        root.left  = helperTree(postorder, left, inorderIdx - 1);
+
+        return root;
+    }
+}