AllAlgorithms
diff --git a/‎algorithms/data-structures/trees/size_of_subtree.py
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+28 b/‎algorithms/data-structures/trees/size_of_subtree.py
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+"""
+    This is an efficient algorithm to find the size of a subtree from every node in O(n) time.
+    The idea is to use one dfs and first calculate the size of subtree of children of a node recursively.
+    Then add the size of each subtree of its children to get the size of its subtree.
+"""
+from collections import defaultdict as dd
+
+
+def dfs(source, parent):
+    # Initial size of root is 1
+    size[source] = 1
+    for child in graph[source]:
+        if child != parent:
+            # Recursively calculate size of subtree of children nodes
+            dfs(child, source)
+            # Adding size of each child's subtree.
+            size[source] += size[child]
+
+
+size = dd(int)
+graph = dd(set)
+n = int(input())
+for i in range(n-1):
+    u, v = map(int, input().split())
+    graph[u].add(v)
+    graph[v].add(u)
+dfs(1, 0)
+print(size)