Binary TreesBinary Trees36
  1. 1Preorder Traversal of a Binary Tree using Recursion
  2. 2Preorder Traversal of a Binary Tree using Iteration
  3. 3Inorder Traversal of a Binary Tree using Recursion
  4. 4Inorder Traversal of a Binary Tree using Iteration
  5. 5Postorder Traversal of a Binary Tree Using Recursion
  6. 6Postorder Traversal of a Binary Tree using Iteration
  7. 7Level Order Traversal of a Binary Tree using Recursion
  8. 8Level Order Traversal of a Binary Tree using Iteration
  9. 9Reverse Level Order Traversal of a Binary Tree using Iteration
  10. 10Reverse Level Order Traversal of a Binary Tree using Recursion
  11. 11Find Height of a Binary Tree
  12. 12Find Diameter of a Binary Tree
  13. 13Find Mirror of a Binary Tree
  14. 14Left View of a Binary Tree
  15. 15Right View of a Binary Tree
  16. 16Top View of a Binary Tree
  17. 17Bottom View of a Binary Tree
  18. 18Zigzag Traversal of a Binary Tree
  19. 19Check if a Binary Tree is Balanced
  20. 20Diagonal Traversal of a Binary Tree
  21. 21Boundary Traversal of a Binary Tree
  22. 22Construct a Binary Tree from a String with Bracket Representation
  23. 23Convert a Binary Tree into a Doubly Linked List
  24. 24Convert a Binary Tree into a Sum Tree
  25. 25Find Minimum Swaps Required to Convert a Binary Tree into a BST
  26. 26Check if a Binary Tree is a Sum Tree
  27. 27Check if All Leaf Nodes are at the Same Level in a Binary Tree
  28. 28Lowest Common Ancestor (LCA) in a Binary Tree
  29. 29Solve the Tree Isomorphism Problem
  30. 30Check if a Binary Tree Contains Duplicate Subtrees of Size 2 or More
  31. 31Check if Two Binary Trees are Mirror Images
  32. 32Calculate the Sum of Nodes on the Longest Path from Root to Leaf in a Binary Tree
  33. 33Print All Paths in a Binary Tree with a Given Sum
  34. 34Find the Distance Between Two Nodes in a Binary Tree
  35. 35Find the kth Ancestor of a Node in a Binary Tree
  36. 36Find All Duplicate Subtrees in a Binary Tree
GraphsGraphs46
  1. 1Breadth-First Search in Graphs
  2. 2Depth-First Search in Graphs
  3. 3Number of Provinces in an Undirected Graph
  4. 4Connected Components in a Matrix
  5. 5Rotten Oranges Problem - BFS in Matrix
  6. 6Flood Fill Algorithm - Graph Based
  7. 7Detect Cycle in an Undirected Graph using DFS
  8. 8Detect Cycle in an Undirected Graph using BFS
  9. 9Distance of Nearest Cell Having 1 - Grid BFS
  10. 10Surrounded Regions in Matrix using Graph Traversal
  11. 11Number of Enclaves in Grid
  12. 12Word Ladder - Shortest Transformation using Graph
  13. 13Word Ladder II - All Shortest Transformation Sequences
  14. 14Number of Distinct Islands using DFS
  15. 15Check if a Graph is Bipartite using DFS
  16. 16Topological Sort Using DFS
  17. 17Topological Sort using Kahn's Algorithm
  18. 18Cycle Detection in Directed Graph using BFS
  19. 19Course Schedule - Task Ordering with Prerequisites
  20. 20Course Schedule 2 - Task Ordering Using Topological Sort
  21. 21Find Eventual Safe States in a Directed Graph
  22. 22Alien Dictionary Character Order
  23. 23Shortest Path in Undirected Graph with Unit Distance
  24. 24Shortest Path in DAG using Topological Sort
  25. 25Dijkstra's Algorithm Using Set - Shortest Path in Graph
  26. 26Dijkstra’s Algorithm Using Priority Queue
  27. 27Shortest Distance in a Binary Maze using BFS
  28. 28Path With Minimum Effort in Grid using Graphs
  29. 29Cheapest Flights Within K Stops - Graph Problem
  30. 30Number of Ways to Reach Destination in Shortest Time - Graph Problem
  31. 31Minimum Multiplications to Reach End - Graph BFS
  32. 32Bellman-Ford Algorithm for Shortest Paths
  33. 33Floyd Warshall Algorithm for All-Pairs Shortest Path
  34. 34Find the City With the Fewest Reachable Neighbours
  35. 35Minimum Spanning Tree in Graphs
  36. 36Prim's Algorithm for Minimum Spanning Tree
  37. 37Disjoint Set (Union-Find) with Union by Rank and Path Compression
  38. 38Kruskal's Algorithm - Minimum Spanning Tree
  39. 39Minimum Operations to Make Network Connected
  40. 40Most Stones Removed with Same Row or Column
  41. 41Accounts Merge Problem using Disjoint Set Union
  42. 42Number of Islands II - Online Queries using DSU
  43. 43Making a Large Island Using DSU
  44. 44Bridges in Graph using Tarjan's Algorithm
  45. 45Articulation Points in Graphs
  46. 46Strongly Connected Components using Kosaraju's Algorithm

Convert a Binary Tree into a Sum Tree

Problem Statement

Given a binary tree, convert it into a Sum Tree. In a Sum Tree, each node is assigned a new value which is the sum of the values of its left and right subtrees in the original tree. The value of a leaf node becomes 0 because it has no children. If the tree is empty, return null or handle it as a base case.

Examples

Input Tree Level Order Output Description
[10, -2, 6, 8, -4, null, 5]
[[13], [4, 5], [0, 0, 0]] Standard tree: Converted to sum tree where each node is the sum of values in its left and right subtrees
[1]
[[0]] Edge case: A single node becomes 0 as it has no children
[] [] Edge case: Empty tree
[5, 3, null, 2, null, null, null, 1]
[[6], [3], [1], [0]] Deep left-skewed tree converted into a sum tree; each node is updated recursively
[7, null, 9, null, null, null, 11]
[[20], [11], [0]] Right-skewed tree: Each node updated to sum of subtree

Solution

Understanding the Problem

The task is to convert a given binary tree into a sum tree. In a sum tree, each node’s value is replaced by the sum of values of its left and right subtrees in the original tree. For leaf nodes, since they don’t have children, their value becomes 0.

This transformation is done recursively, which means for each node, we first compute the sum of its left and right subtrees and update the node’s value, then repeat the process for its children.

Step-by-Step Solution with Example

Step 1: Understand Tree Structure

Consider this example binary tree:


        10
       /        -2    6
     /    /     8  -4 7   5

We need to convert this into a sum tree. For every node, we’ll replace its value with the sum of values in its left and right subtrees from the original tree.

Step 2: Process Leaf Nodes

Leaf nodes have no children, so their new value becomes 0.

  • 8 → 0
  • -4 → 0
  • 7 → 0
  • 5 → 0

Step 3: Process Intermediate Nodes

Now, we go one level up and calculate the new values based on the original values of children.

  • -2 has children 8 and -4 → new value = 8 + (-4) = 4
  • 6 has children 7 and 5 → new value = 7 + 5 = 12

Step 4: Process Root Node

Finally, we compute the value for the root node.

  • 10 has children -2 and 6 → in original tree, their values were -2 and 6 → new value = -2 + 6 = 4

Step 5: Update Tree

After the transformation, the sum tree looks like this:


        4
       /        4    12
     /    /     0   0 0   0

Edge Cases

Case 1: Tree with Multiple Levels

This is the general case like the example above. Each internal node gets updated based on the sum of its children’s original values. Leaf nodes become 0.

Case 2: Tree with a Single Node

If the tree only has one node, there are no children to sum. Therefore, the single node becomes 0, since it's considered a leaf node.

Case 3: Empty Tree

If the input tree is empty (i.e., the root is null), then the output is also an empty tree or null. This is the base case in our recursion.

Finally

Converting a binary tree to a sum tree requires a post-order traversal, where we visit left and right children before updating the current node. The solution handles all edge cases and maintains correctness by updating nodes only after their subtrees are processed. Always remember to handle base cases like leaf nodes and empty trees to avoid runtime errors.

Algorithm Steps

  1. If the tree is empty, return 0.
  2. Recursively convert the left subtree into a sum tree and obtain its sum.
  3. Recursively convert the right subtree into a sum tree and obtain its sum.
  4. Store the current node's original value in a temporary variable, e.g., oldVal.
  5. Update the current node's value to the sum of the left and right subtree sums.
  6. Return the sum of the updated node value and the original value (node.val + oldVal) so it can be used by the parent node.

Code

Python
Java
JavaScript
C
C++
C#
Kotlin
Swift
Go
Php
class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def toSumTree(root):
    if root is None:
        return 0
    old_val = root.val
    left_sum = toSumTree(root.left)
    right_sum = toSumTree(root.right)
    root.val = left_sum + right_sum
    return root.val + old_val

# Example usage:
if __name__ == '__main__':
    # Construct the binary tree:
    #         10
    #        /  \
    #      -2    6
    #      / \   
    #     8  -4  
    root = TreeNode(10, TreeNode(-2, TreeNode(8), TreeNode(-4)), TreeNode(6))
    toSumTree(root)
    # The tree is now converted into a sum tree

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