Completed Trees-2 Assignemt

postorder_inorder_contruct_tree.java

@@ -0,0 +1,42 @@
+// time complexity: O(n)
+// space complexity : O(n)
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+    int idx;
+    public TreeNode buildTree(int[] inorder, int[] postorder) {
+        HashMap<Integer, Integer> map = new HashMap<>();
+        for(int i=0; i<inorder.length; i++){
+            map.put(inorder[i],i);
+        }
+        this.idx=postorder.length-1;
+        return helper(postorder,0,postorder.length-1,map);
+    }
+    private TreeNode helper(int[] postorder, int st, int end, HashMap<Integer, Integer> map){
+        //base 
+        if(st>end) return null; //--> this works because we need null in cases the values are missing 
+        //logic
+        int rootVal = postorder[idx];
+        idx--;
+        TreeNode root= new TreeNode(rootVal);
+        int rootIdx= map.get(rootVal);
+        //right
+        root.right = helper(postorder,rootIdx+1,end,map); 
+        //left
+        root.left = helper(postorder,st, rootIdx-1,map); 
+        return root;
+    }
+}

sum_root_to_leaf.java

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+//time comp : O(n)
+//space comp: O(1)
+//using recusrion to parse to each node of tree and add the values in the result which is a global variable 
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+    int res;
+    public int sumNumbers(TreeNode root) {
+        helper(root,0);
+        return res;
+    }
+    private void helper(TreeNode root, int curr){
+        //base
+        if(root==null) return;
+        // logic 
+        curr = curr*10+ root.val;
+        if(root.left==null && root.right==null){
+            res+=curr;
+        }
+        helper(root.left,curr);
+        helper(root.right,curr);
+
+    }
+}