Path Sum Root to Leaf in Binary Tree in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the current node is a leaf node.

DSA Rust

if node.left.is_none() && node.[1].is_none() {

Drag options to blanks, or click blank then click option'

Aparent

Bleft

Cchild

Dright

Attempts:

3 left

💡 Hint

Common Mistakes

Checking only one child instead of both.

Using incorrect field names like 'child' or 'parent'.

A leaf node has no left or right children, so both must be None.

2fill in blank

medium

Complete the code to subtract the current node's value from the target sum.

DSA Rust

let remaining_sum = target_sum - node.[1];

Drag options to blanks, or click blank then click option'

Adata

Bvalue

Cval

Dkey

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'value' or 'data' which are not defined in this struct.

Using 'key' which is unrelated.

The node's value is stored in the 'val' field in this tree structure.

3fill in blank

hard

Fix the error in the recursive call to check left subtree.

DSA Rust

if let Some(left_node) = &node.[1] {
    if has_path_sum(left_node, remaining_sum) {
        return true;
    }
}

Drag options to blanks, or click blank then click option'

Aright

Bleft

Cchild

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'right' instead of 'left' for left subtree.

Using undefined fields like 'child' or 'parent'.

To check the left subtree, we must access the 'left' child node.

4fill in blank

hard

Fill both blanks to check the right subtree recursively and return true if path found.

DSA Rust

if let Some([1]_node) = &node.[2] {
    if has_path_sum([1]_node, remaining_sum) {
        return true;
    }
}

Drag options to blanks, or click blank then click option'

Aright

Bleft

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing 'left' and 'right' in variable and field names.

Using undefined field names.

We name the variable 'right_node' and access the 'right' child to check the right subtree.

5fill in blank

hard

Fill all three blanks to complete the function signature and return false if no path found.

DSA Rust

fn [1](node: &Option<Box<TreeNode>>, [2]: i32) -> bool {
    if node.is_none() {
        return [3];
    }
    // function body continues...

Drag options to blanks, or click blank then click option'

Ahas_path_sum

Btarget_sum

Cfalse

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Returning true when node is None.

Using wrong parameter names or function names.

The function is named 'has_path_sum', takes 'target_sum' as parameter, and returns false if node is None.

Practice

1. What does the "Path Sum Root to Leaf" problem ask you to find in a binary tree?

easy

A. The maximum depth of the binary tree

B. If there exists a root-to-leaf path with a sum equal to a target value

C. The total number of nodes in the binary tree

D. The sum of all node values in the binary tree

Solution

1. Step 1: Understand the problem statement

   The problem asks if there is any path from the root node down to any leaf node where the sum of the node values equals a given target sum.

2. Step 2: Differentiate from other tree properties

   This is different from finding the maximum depth, counting nodes, or summing all nodes. It specifically checks for a path sum.

3. Final Answer:

   If there exists a root-to-leaf path with a sum equal to a target value -> Option B

4. Quick Check:

   Path sum means root-to-leaf sum check = A [OK]

Hint: Focus on root-to-leaf paths and their sums [OK]

Common Mistakes:

• Confusing path sum with total tree sum
• Checking any path, not just root-to-leaf
• Mixing depth or node count with sum

2. Which of the following Rust function signatures correctly represents a function to check path sum in a binary tree node defined as struct TreeNode { val: i32, left: Option, right: Option }?

easy

A. fn has_path_sum(root: Option<TreeNode>, sum: i32) -> bool

B. fn has_path_sum(root: TreeNode, sum: i32) -> i32

C. fn has_path_sum(root: Option, sum: i32) -> bool

D. fn has_path_sum(root: &TreeNode, sum: i32) -> Option<i32>

Solution

1. Step 1: Analyze the tree node ownership and option usage

   The tree nodes are wrapped in Option to allow for empty children and heap allocation. The function should accept Option to handle empty trees.

2. Step 2: Check return type and parameters

   The function returns a boolean indicating if a path sum exists. So the signature fn has_path_sum(root: Option, sum: i32) -> bool matches the problem.

3. Final Answer:

   fn has_path_sum(root: Option, sum: i32) -> bool -> Option C

4. Quick Check:

   Correct ownership and return type = D [OK]

Hint: Use Option for nullable tree nodes [OK]

Common Mistakes:

• Using TreeNode directly without Option or Box
• Wrong return type (not bool)
• Passing references incorrectly

3. Given the following Rust code, what is the output?

struct TreeNode {
    val: i32,
    left: Option,
    right: Option,
}

fn has_path_sum(root: Option, sum: i32) -> bool {
    match root {
        None => false,
        Some(node) => {
            if node.left.is_none() && node.right.is_none() {
                return node.val == sum;
            }
            has_path_sum(node.left, sum - node.val) || has_path_sum(node.right, sum - node.val)
        }
    }
}

fn main() {
    let leaf = TreeNode { val: 5, left: None, right: None };
    let root = TreeNode {
        val: 4,
        left: Some(Box::new(leaf)),
        right: None,
    };
    println!("{}", has_path_sum(Some(Box::new(root)), 9));
}

medium

A. Compilation error

B. false

C. Runtime panic

D. true

Solution

1. Step 1: Trace the path sum calculation

   The tree has root 4 with left child 5. The target sum is 9. The path 4 -> 5 sums to 9, so the function returns true.

2. Step 2: Confirm leaf node check and recursion

   At leaf node 5, it checks if 5 == 9 - 4 = 5, which is true, so returns true.

3. Final Answer:

   true -> Option D

4. Quick Check:

   4 + 5 = 9 matches sum = true [OK]

Hint: Check leaf node sums carefully [OK]

Common Mistakes:

• Ignoring leaf node condition
• Subtracting sum incorrectly
• Confusing Option handling

4. The following Rust function is intended to check if a path sum exists but has a bug. What is the bug?

fn has_path_sum(root: Option, sum: i32) -> bool {
    if root.is_none() {
        return false;
    }
    let node = root.unwrap();
    if node.left.is_none() && node.right.is_none() {
        return node.val == sum;
    }
    has_path_sum(node.left, sum - node.val) && has_path_sum(node.right, sum - node.val)
}

medium

A. Using && instead of || for recursive calls

B. Unwrapping Option without checking None

C. Not handling empty tree case

D. Incorrect sum subtraction

Solution

1. Step 1: Analyze recursive call logic

   The function uses logical AND (&&) to combine left and right subtree results, but the problem requires checking if any path exists, so logical OR (||) is correct.

2. Step 2: Understand impact of && vs ||

   Using && means both left and right paths must have the sum, which is incorrect. Only one path needs to match.

3. Final Answer:

   Using && instead of || for recursive calls -> Option A

4. Quick Check:

   Path sum needs OR (||), not AND (&&) [OK]

Hint: Use || to check any path, not all paths [OK]

Common Mistakes:

• Using && instead of ||
• Unwrapping Option without safe check
• Ignoring leaf node condition

5. You want to modify the path sum function to return all root-to-leaf paths where the sum equals the target. Which approach is best to implement this in Rust?

hard

A. Use backtracking with a mutable vector to store current path and a vector of vectors for results

B. Use a global variable to store paths and update it in recursion

C. Return a boolean and print paths inside the function

D. Use iterative BFS traversal without recursion

Solution

1. Step 1: Understand the requirement for all paths

   To find all root-to-leaf paths with a given sum, we need to explore all paths and store those matching the sum.

2. Step 2: Choose backtracking with mutable vectors

   Backtracking allows building paths step-by-step, storing current path in a mutable vector, and adding copies to results when sum matches.

3. Final Answer:

   Use backtracking with a mutable vector to store current path and a vector of vectors for results -> Option A

4. Quick Check:

   Backtracking with path storage = B [OK]

Hint: Backtrack paths, store copies when sum matches [OK]

Common Mistakes:

• Using global variables (not idiomatic in Rust)
• Printing instead of returning paths
• Trying BFS which is complex for path tracking