[feat ]: leetcode 310

Signed-off-by: Bo-Wei Chen(BWbwchen) <tim.chenbw@gmail.com>

Author: BWbwchen

leetcode/0310_Minimum-Height-Trees.cpp

@@ -0,0 +1,50 @@
+class Solution {
+public:
+    vector<int> findMinHeightTrees(int n, vector<vector<int>>& edges) {
+        // Since there are at most 2 nodes as the centre(A tree node will have at most 2 neighbor, If 3 centre, the middle one will become new centre).
+        // How to find the centre?
+        // remove the leaf node layer by layer, the remain 1 or 2 nodes are the centre.
+        // How to detect a node is a leaf node or not?
+        // keep tracking the edge of each node!
+        if (n == 1)
+            return {0};
+
+        vector<int> in_degree(n, 0);
+        vector<vector<int>> graph(n);
+        for (auto e : edges) {
+            in_degree[e[0]]++;
+            in_degree[e[1]]++;
+            graph[e[0]].push_back(e[1]);
+            graph[e[1]].push_back(e[0]);
+        }
+
+        queue<int> q;
+        for (int node = 0; node < in_degree.size(); ++node) {
+            if (in_degree[node] == 1) // leaf node
+                q.push(node);
+        }
+
+        int leaf_node = 0;
+        while(n - leaf_node > 2) { // at most 2 centres
+            // remove leaf node
+            int tmp_leaf_node = q.size();
+            leaf_node += tmp_leaf_node;
+            for (int i = 0; i < tmp_leaf_node; ++i) {
+                int leaf = q.front();
+                q.pop();
+
+                for (auto neighbor : graph[leaf]) {
+                    if (--in_degree[neighbor] == 1) 
+                        q.push(neighbor);
+                }
+            }
+        }
+
+        vector<int> ret;
+        while(!q.empty()) {
+            ret.push_back(q.front());
+            q.pop();
+        }
+        return ret;
+    }
+};