2533: reduce time, memory by using BFS

yous · Sep 23, 2021 · 912bca2 · 912bca2
1 parent b3a60e7
commit 912bca2
Showing 1 changed file with 37 additions and 21 deletions.
diff --git a/problem/2533/main.cpp b/problem/2533/main.cpp
@@ -7,24 +7,37 @@ using namespace std;
 
 int N;
 vector<vector<int>> ADJ;
-vector<pair<int, int>> depths;
-vector<int> leaves;
+vector<int> nodes;
+vector<bool> leaves;
 vector<int> parents;
 vector<bool> early;
 
-void dfs(int u, int s, int depth) {
-    depths.emplace_back(-depth, u);
-    bool has_child = false;
-    for (auto v : ADJ[u]) {
-        if (v == s) {
-            continue;
+void bfs(void) {
+    queue<pair<int, int>> qu;
+    parents[0] = -1;
+    nodes.emplace_back(0);
+    qu.emplace(0, -1);
+    int depth = 1;
+    while (!qu.empty()) {
+        int sz = qu.size();
+        while (sz-- > 0) {
+            auto [u, s] = qu.front();
+            qu.pop();
+            bool has_child = false;
+            for (auto v : ADJ[u]) {
+                if (v == s) {
+                    continue;
+                }
+                has_child = true;
+                parents[v] = u;
+                nodes.emplace_back(v);
+                qu.emplace(v, u);
+            }
+            if (!has_child) {
+                leaves[u] = true;
+            }
         }
-        has_child = true;
-        parents[v] = u;
-        dfs(v, u, depth + 1);
-    }
-    if (!has_child) {
-        leaves.emplace_back(u);
+        depth++;
     }
 }
 
@@ -33,6 +46,7 @@ int main() {
     cin.tie(nullptr);
     cin >> N;
     ADJ.resize(N);
+    leaves.resize(N);
     parents.resize(N);
     early.resize(N);
     for (int i = 0; i < N - 1; i++) {
@@ -41,17 +55,19 @@ int main() {
         ADJ[u - 1].emplace_back(v - 1);
         ADJ[v - 1].emplace_back(u - 1);
     }
-    parents[0] = -1;
-    dfs(0, -1, 0);
+    bfs();
     int ans = 0;
-    for (auto leaf : leaves) {
-        if (!early[parents[leaf]]) {
-            early[parents[leaf]] = true;
+    for (int u = 0; u < N; u++) {
+        if (!leaves[u]) {
+            continue;
+        }
+        if (!early[parents[u]]) {
+            early[parents[u]] = true;
             ans++;
         }
     }
-    sort(depths.begin(), depths.end());
-    for (auto [depth, u] : depths) {
+    for (auto it = nodes.rbegin(); it != nodes.rend(); ++it) {
+        int u = *it;
         if (early[u]) {
             continue;
         }