Create trees-3.java

trees-3.java

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+// Approach:
+// 1. Perform a DFS traversal while maintaining the current path and its sum.
+// 2. Add the current node's value to the path and update the running sum.
+// 3. If the current node is a leaf and the running sum equals the target sum,
+//    add a copy of the current path to the result.
+// 4. Recursively explore the left and right subtrees.
+// 5. After visiting both subtrees, remove the current node from the path
+//    (backtracking) to explore other paths.
+
+// Time Complexity: O(n)
+// Space Complexity: O(h)
+// h = height of the tree (excluding the output list)
+class Solution {
+    List<List<Integer>> res;
+    public List<List<Integer>> pathSum(TreeNode root, int targetSum) {
+         this.res=new ArrayList<>();
+        helper(root,targetSum,0,new ArrayList<>());
+        return res;
+    }
+    private void helper(TreeNode root,int targetSum,int currentsum,List<Integer> path){
+        if(root==null) return;
+        currentsum+=root.val;
+        path.add(root.val);
+        if(root.left==null && root.right==null){
+            if(currentsum==targetSum){
+                res.add(new ArrayList<>(path));
+            }
+        }
+        helper(root.left,targetSum,currentsum,path);
+        helper(root.right,targetSum,currentsum,path);
+        path.remove(path.size()-1);
+    }
+}
+//problem-2
+// Approach:
+// 1. Compare the left and right subtrees recursively.
+// 2. If both nodes are null, they are symmetric.
+// 3. If one node is null or their values differ, return false.
+// 4. Compare the left child of the left subtree with the right child of the right subtree.
+// 5. Compare the right child of the left subtree with the left child of the right subtree.
+// 6. The tree is symmetric only if both recursive comparisons return true.
+
+// Time Complexity: O(n)
+// Space Complexity: O(h)
+// h = height of the tree (recursion stack)
+class Solution {
+    public boolean isSymmetric(TreeNode root) {
+        if(root==null) return true;
+        return helper(root.left,root.right);
+    }
+    private boolean helper(TreeNode left,TreeNode right){
+        if(left==null && right==null) return true;
+        if(left==null||right==null) return false;
+        if(left.val!=right.val) return false;
+        return helper(left.left,right.right) && helper(left.right,right.left);
+    }
+}