House Robber 3

C++/Sum-of-distances-in-tree.cpp

@@ -0,0 +1,66 @@
+class Solution
+{
+public:
+    /*
+    Uses classics Tree Re-rooting DP,
+    Need to do 2 traversals: DFSes but 1 as Post Order and second as Pre-order
+
+    When we move our root from parent to its child i, 
+    count[i] points get 1 closer to root, n - count[i] nodes get 1 futhur to root.
+
+    Time Complexity: O(N)  because both DFS visit all ndoes
+    Space Compexity: O(N)  because of the resultant & helper subtree-node-count array
+
+    */
+
+    /* Post Order to update distances while going from parent to root
+    To track a count of number of children in the subtree
+    */
+    void dfs1(int node, int parent, vector<int> &res, vector<int> &count, vector<vector<int>> &graph)
+    {
+        for (auto child : graph[node])
+        {
+            if (child == parent)
+                continue;
+            dfs1(child, node, res, count, graph);
+            count[node] += count[child];
+            res[node] += res[child] + count[child];
+        }
+    }
+
+    /* Pre order traversal style DFS, to update the result vector in direction of child -> parent */
+    void dfs2(int node, int parent, vector<int> &res, vector<int> &count, vector<vector<int>> &graph, int n)
+    {
+        for (auto child : graph[node])
+        {
+            if (child == parent)
+                continue;
+            res[child] = res[node] - count[child] + n - count[child];
+            dfs2(child, node, res, count, graph, n);
+        }
+    }
+
+    vector<int> sumOfDistancesInTree(int n, vector<vector<int>> &edges)
+    {
+
+        vector<int> count(n, 1); //stores count of number of children, atleast 1 node, ie itself should be in count[node]
+        vector<int> res(n, 0);   //Stores final distance sum for each node
+
+        vector<vector<int>> graph(n);
+
+        int u, v;
+
+        // Constructing the graph from the given array
+        for (int i = 0; i < edges.size(); ++i)
+        {
+            u = edges[i][0], v = edges[i][1];
+            graph[u].push_back(v);
+            graph[v].push_back(u);
+        }
+
+        dfs1(0, -1, res, count, graph);
+        dfs2(0, -1, res, count, graph, n);
+
+        return res;
+    }
+};

README.md

@@ -150,6 +150,7 @@ Check out ---> [Sample PR](https://github.com/codedecks-in/LeetCode-Solutions/pu
 | 54     | [Spiral Matrix](https://leetcode.com/problems/spiral-matrix/)                                                         | [C++](./C++/Spiral-matrix.cpp)                                                                     | O(M\*N)      | O(M\*N)         | Medium     | Array              |
 | 238     | [Product of Array Except Self](https://leetcode.com/problems/product-of-array-except-self/)                                                         | [C++](./C++/238.Product_of_array_except_self)                                                                   | O(N)      | O(N)         | Medium     | Array              |
 
+
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@@ -396,7 +397,7 @@ Check out ---> [Sample PR](https://github.com/codedecks-in/LeetCode-Solutions/pu
 | 070  | [Climbing Stairs](https://leetcode.com/problems/climbing-stairs/)                                                   | [Java](./Java/climbing-stairs.java)                       | _O(N)_    | _O(1)_    | Easy       | DP                      |      |
 | 730  | [Count Different Palindromic Subsequences](https://leetcode.com/problems/count-different-palindromic-subsequences/) | [C++](./C++/Count-Different-Palindromic-Subsequences.cpp) | _O(N\*N)_ | _O(N\*N)_ | Hard       | DP                      |      |
 | 55   | [Jump Game](https://leetcode.com/problems/jump-game/)                                                               | [Python](./Python/jumpGame.py)                            | _O(N)_    | _O(N)_    | Medium     | DP                      |      |
-| 337  | [House Robber 3](https://leetcode.com/problems/house-robber-iii/)                                                   | [C++](./C++/House-Robber-3.cpp)                           | _O(n)_    | _O(n)_    | Medium     | DP Tree DFS Binary Tree |
+| 337  | [House Robber 3](https://leetcode.com/problems/house-robber-iii/)                                                   | [C++](./C++/House-Robber-3.cpp)                           | _O(n)_    | _O(n)_    | Medium     | DP Tree DFS Binary Tree |      |
 
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