fix 101: refactor code

Author: novahe

leetcode/0101.Symmetric-Tree/101. Symmetric Tree.go

@@ -17,31 +17,18 @@ type TreeNode = structures.TreeNode
  */
 
 func isSymmetric(root *TreeNode) bool {
-	if root == nil {
-		return true
-	}
-	return isSameTree(invertTree(root.Left), root.Right)
-}
-
-func isSameTree(p *TreeNode, q *TreeNode) bool {
-	if p == nil && q == nil {
-		return true
-	} else if p != nil && q != nil {
-		if p.Val != q.Val {
+	var dfs func(rootLeft, rootRight *TreeNode) bool
+	dfs = func(rootLeft, rootRight *TreeNode) bool {
+		if rootLeft == nil && rootRight == nil {
+			return true
+		}
+		if rootLeft == nil || rootRight == nil {
 			return false
 		}
-		return isSameTree(p.Left, q.Left) && isSameTree(p.Right, q.Right)
-	} else {
-		return false
-	}
-}
-
-func invertTree(root *TreeNode) *TreeNode {
-	if root == nil {
-		return nil
+		if rootLeft.Val != rootRight.Val {
+			return false
+		}
+		return dfs(rootLeft.Left, rootRight.Right) && dfs(rootLeft.Right, rootRight.Left)
 	}
-	invertTree(root.Left)
-	invertTree(root.Right)
-	root.Left, root.Right = root.Right, root.Left
-	return root
+	return root == nil || dfs(root.Left, root.Right)
 }