day 107: add a much faster O(N) solution for level-wise binary tree

day107/problem.go

@@ -44,3 +44,24 @@ func max(a, b int) int {
 	}
 	return b
 }
+
+// BinaryTreeByLevelFaster prints the binary tree level-wise faster than O(N^2).
+// This version runs in O(N) with O(N) extra space.
+func BinaryTreeByLevelFaster(tree *BinaryTree) []int {
+	var result []int
+	level := []*BinaryTree{tree}
+	for len(level) != 0 {
+		nextLevel := make([]*BinaryTree, 0, 2*len(level))
+		for _, node := range level {
+			result = append(result, node.Value)
+			if node.Left != nil {
+				nextLevel = append(nextLevel, node.Left)
+			}
+			if node.Right != nil {
+				nextLevel = append(nextLevel, node.Right)
+			}
+		}
+		level = nextLevel
+	}
+	return result
+}

day107/problem_test.go

@@ -35,3 +35,20 @@ func BenchmarkBinaryTreeByLevel(b *testing.B) {
 		}
 	}
 }
+
+func TestBinaryTreeByLevelFaster(t *testing.T) {
+	t.Parallel()
+	for _, tc := range testcases {
+		if result := BinaryTreeByLevelFaster(tc.tree); !reflect.DeepEqual(tc.expected, result) {
+			t.Errorf("Expected %v got %v", tc.expected, result)
+		}
+	}
+}
+
+func BenchmarkBinaryTreeByLevelFaster(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			BinaryTreeByLevelFaster(tc.tree)
+		}
+	}
+}