🚀Day 9. K Sum Paths 🧠

The problem can be found at the following link: Question Link

💡 Problem Description:

Given a binary tree and an integer k, determine the number of downward-only paths where the sum of the node values in the path equals k. A path can start and end at any node within the tree but must always move downward (from parent to child).

🔍 Example Walkthrough:

Example 1:

Input:

       1
      / \
     2   3

k = 3

Output:

2

Explanation:

• Path 1: 1 → 2 (Sum = 3)

• Path 2: 3 (Sum = 3)

Example 2:

Input: k = 7

         8
       /   \
      4     5
     / \     \
    3   2     2
   / \   \
  3  -2   1

Output:

3

Explanation:

The following paths sum to k

Constraints

• 1 ≤ number of nodes ≤ $10^4$

• -100 ≤ node value ≤ 100

• -109 ≤ k ≤ $10^9$

🎯 My Approach:

Prefix Sum DFS Approach

1. Traverse the Tree (DFS): Use depth-first search (DFS) to traverse the binary tree. As we move from the root to each node, maintain a running sum of node values.

2. Maintain Prefix Sum Frequencies: Use a hash map (or dictionary) to store the frequency of each prefix sum encountered along the current path.

   • Key: The prefix sum value.

   • Value: The number of times this prefix sum has occurred.

3. Check for Valid Paths: For each node visited, compute the current running sum. Then check if (current sum - k) exists in the hash map.

   • If it does, it indicates that there is a valid subpath ending at the current node whose sum equals k.

   • Increase the count by the frequency of (current sum - k).

4. Recurse and Backtrack:

   • Recursively process the left and right children with the updated running sum.

   • After processing both subtrees, decrement the frequency of the current prefix sum in the hash map to backtrack (ensuring that paths in other branches are not affected).

🕒 Time and Auxiliary Space Complexity

Expected Time Complexity: O(N), as each node is visited exactly once. Expected Auxiliary Space Complexity: O(H + N), where H is the height of the tree (for recursion) and N for the hash map.

📝 Solution Code

Code (C++)

class Solution {
public:
    unordered_map<int, int> m;
    int cnt = 0;
    void dfs(Node* node, int k, int currSum) {
        if (!node) return;
        currSum += node->data;
        if (m.count(currSum - k))
            cnt += m[currSum - k];
        m[currSum]++;
        dfs(node->left, k, currSum);
        dfs(node->right, k, currSum);
        m[currSum]--;
    }
    int sumK(Node* root, int k) {
        m[0] = 1;
        dfs(root, k, 0);
        return cnt;
    }
};

🌲 Alternative Approaches

2️⃣ Brute Force Recursive Approach

class Solution {
public:
    int countFrom(Node* node, int k) {
        if (!node) return 0;
        return (node->data == k) +
               countFrom(node->left, k - node->data) +
               countFrom(node->right, k - node->data);
    }
    int sumK(Node* root, int k) {
        if (!root) return 0;
        return countFrom(root, k) + sumK(root->left, k) + sumK(root->right, k);
    }
};

Note: This approach recalculates many subpaths and is too slow for large trees.

• ✅ Optimized than brute force but slower than the prefix sum approach

• ⚠️ Still has overlapping subproblems (may hit TLE in large trees).

  Note: Not Passing All Test Cases

• Test Cases Passed:

• 1110 /1120

• Time limit exceeded.

---

3️⃣ Iterative DFS with Path Vector

class Solution {
public:
    int sumK(Node* root, int k) {
        if (!root) return 0;
        int cnt = 0;
        stack<pair<Node*, vector<int>>> st;
        st.push({root, {}});
        while (!st.empty()) {
            auto cur = st.top(); st.pop();
            Node* node = cur.first;
            vector<int> path = cur.second;
            path.push_back(node->data);
            int s = 0;
            for (int i = path.size() - 1; i >= 0; i--) {
                s += path[i];
                if (s == k) cnt++;
            }
            if (node->right) st.push({node->right, path});
            if (node->left) st.push({node->left, path});
        }
        return cnt;
    }
};

Note: This method tracks the full path from the root, which leads to high memory usage and slower runtime.

• 🔥 Iterative implementation avoids recursion stack overflow.

• ⚠️ Higher space complexity due to storing entire paths.

📊 Comparison of Approaches

Approach	Time Complexity	Space Complexity	Method	Pros	Cons
1️⃣ Prefix Sum (Optimized)	🟢 O(N)	🟡 O(H + N)	DFS + HashMap	Fastest, avoids redundant work	Slightly complex to implement
2️⃣ Count from Each Node (Recursive)	🟡 O(N²)	🟡 O(H)	Pure Recursion	Simple, intuitive	Overlapping subproblems, slower
3️⃣ Iterative DFS (Path Tracking)	🟡 O(N²)	🔴 O(N·H)	Stack-based	No recursion depth issues	High memory usage for large paths

💡 Best Choice?

• Balanced Trees / Small Input: ✅ Approach 1 (Prefix Sum) is the fastest. For optimal performance on large trees, use the Prefix Sum DFS approach.

• Unbalanced / Deep Trees: ✅ Approach 3 (Iterative DFS) avoids stack overflow.

• Simple Understanding: ✅ Approach 2 helps with conceptual clarity but may be slow.

Code (Java)

class Solution {
    Map<Integer,Integer> m = new HashMap<>();
    int cnt = 0;
    void dfs(Node r, int k, int s) {
        if(r==null)return;
        s += r.data;
        if(m.containsKey(s-k)) cnt += m.get(s-k);
        m.put(s, m.getOrDefault(s,0)+1);
        dfs(r.left, k, s);
        dfs(r.right, k, s);
        m.put(s, m.get(s)-1);
        if(m.get(s)==0)m.remove(s);
    }
    public int sumK(Node r, int k) {
        m.put(0,1);
        dfs(r, k, 0);
        return cnt;
    }
}

Code (Python)

class Solution:
    def sumK(self, r, k):
        self.c = 0
        m = {0: 1}
        def dfs(r, s):
            if not r: return
            s += r.data
            self.c += m.get(s - k, 0)
            m[s] = m.get(s, 0) + 1
            dfs(r.left, s)
            dfs(r.right, s)
            m[s] -= 1
            if m[s] == 0: del m[s]
        dfs(r, 0)
        return self.c

🎯 Contribution and Support:

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