Solve the "Test if a binary tree is symmetric" problem

README.md

@@ -124,7 +124,7 @@ Binary Trees
 | Problem                                                                  | Test         | Implemented |
 |--------------------------------------------------------------------------|:------------:|:-----------:|
 | [Test if a binary tree is balanced][144]                                 | [tests][145] |      ✓      |
-| [Test if a binary tree is symmetric][146]                                | [tests][147] |             |
+| [Test if a binary tree is symmetric][146]                                | [tests][147] |      ✓      |
 | [Compute the lowest common ancestor in a binary tree][148]               | [tests][149] |             |
 | [Compute the LCA when nodes have parent pointers][150]                   | [tests][151] |             |
 | [Sum the root-to-leaf paths in a binary tree][152]                       | [tests][153] |             |
@@ -542,8 +542,8 @@ Honors Class
 [143]: in_progress.md
 [144]: btrees/balanced.go
 [145]: btrees/balanced_test.go
-[146]: in_progress.md
-[147]: in_progress.md
+[146]: btrees/symmetric.go
+[147]: btrees/symmetric_test.go
 [148]: in_progress.md
 [149]: in_progress.md
 [150]: in_progress.md

btrees/symmetric.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2015, Peter Mrekaj. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE.txt file.
+
+package btrees
+
+// checkSymmetry returns true if the l binary
+// tree is symmetric to the r binary tree.
+func checkSymmetry(l, r *BTree) bool {
+	switch {
+	case l == nil && r == nil:
+		return true
+	case l != nil && r != nil:
+		return l.Data == r.Data &&
+			checkSymmetry(l.left, r.right) &&
+			checkSymmetry(l.right, r.left)
+	}
+	return false
+}
+
+// IsSymmetric returns true if t is a symmetric binary tree.
+func IsSymmetric(t *BTree) bool {
+	if t == nil {
+		return true
+	}
+	return checkSymmetry(t.left, t.right)
+}

btrees/symmetric_test.go

@@ -0,0 +1,61 @@
+// Copyright (c) 2015, Peter Mrekaj. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE.txt file.
+
+package btrees
+
+import "testing"
+
+func TestIsSymmetric(t *testing.T) {
+	for _, test := range []struct {
+		tree *BTree
+		want bool
+	}{
+		// Symmetric binary trees.
+		{(*BTree)(nil), true},
+		{&BTree{"TREE_0", nil, nil}, true},
+		{&BTree{"TREE_1",
+			&BTree{"A",
+				&BTree{"B",
+					nil,
+					&BTree{"C", nil, nil}},
+				nil},
+			&BTree{"A",
+				nil,
+				&BTree{"B",
+					&BTree{"C", nil, nil},
+					nil}}}, true},
+
+		// Non-symmetric binary trees.
+		{&BTree{"TREE_2", &BTree{"A", nil, nil}, nil}, false},
+		{&BTree{"TREE_3", nil, &BTree{"A", nil, nil}}, false},
+		{&BTree{"TREE_4", &BTree{"A", nil, nil}, &BTree{"B", nil, nil}}, false},
+		{&BTree{"TREE_5",
+			&BTree{"A",
+				&BTree{"B", nil, nil},
+				nil},
+			&BTree{"C", nil, nil}}, false},
+	} {
+		if got := IsSymmetric(test.tree); got != test.want {
+			t.Errorf("IsSymmetric(%v) = %t; want %t", test.tree, got, test.want)
+		}
+	}
+}
+
+func BenchmarkIsSymmetric(b *testing.B) {
+	tree := &BTree{"ROOT",
+		&BTree{"A",
+			&BTree{"B",
+				nil,
+				&BTree{"C", nil, nil}},
+			nil},
+		&BTree{"A",
+			nil,
+			&BTree{"B",
+				&BTree{"C", nil, nil},
+				nil}}}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		IsSymmetric(tree)
+	}
+}