trees2 HW

BTinorderPostorder

@@ -0,0 +1,31 @@
+//O(n) TC
+// O(n) SC
+class Solution {
+     HashMap<Integer,Integer> hm ;
+        int postIdx;
+    public TreeNode buildTree(int[] inorder, int[] postorder) {
+        this.hm=new HashMap();
+        postIdx=postorder.length-1;
+        for(int i=0;i<postorder.length;i++){
+            hm.put(inorder[i],i);
+        }
+
+       return helper(postorder,0,postorder.length-1);
+
+    }
+    private TreeNode helper(int[] postorder,int st,int end){
+        if(st>end) return null;
+
+         int rootVal=postorder[postIdx];
+        int rootIdx=hm.get(rootVal);
+
+        postIdx--;
+
+        TreeNode root=new TreeNode(rootVal);
+        root.right=helper(postorder,rootIdx+1,end);
+        root.left=helper(postorder,st,rootIdx-1);
+        return root;
+
+
+    }
+}

rootToLeafSum

@@ -0,0 +1,34 @@
+//TC :- O(n)
+ //SC:- O(h)
+
+
+class Solution {
+    int xsum = 0;
+
+    public int sumNumbers(TreeNode root) {
+        if (root == null)
+            return xsum;
+        helper(root, 0);
+        return xsum;
+
+    }
+
+    private void helper(TreeNode root, int sum) {
+        if (root == null) {
+
+            return;
+        }
+        sum=10 * sum + root.val;
+        if (root.left == null && root.right == null) {
+
+            xsum = xsum + sum;
+        }
+
+        helper(root.left, sum);
+
+
+        helper(root.right, sum);
+
+
+    }
+}