LeetCode101----对称二叉树

给定一个二叉树，检查它是否是镜像对称的。

例如，二叉树 [1,2,2,3,4,4,3] 是对称的。

    1
   / 
  2   2
 /  / 
3  4 4  3

但是下面这个 [1,2,2,null,3,null,3] 则不是镜像对称的:

    1
   / 
  2   2
      
   3    3

代码如下：

public class LeetCode101 {
	public boolean isSymmetric = true;

	public static class TreeNode {
		int val;
		TreeNode left;
		TreeNode right;

		TreeNode(int x) {
			val = x;
		}
	}

	public boolean isSymmetric(TreeNode root) {
		if (root == null) {
			return true;
		}
		isSymmetric(root.left, root.right);
		return isSymmetric;
	}

	public void isSymmetric(TreeNode left, TreeNode right) {
		if (left == null || right == null) {
			if (left != right) {
				isSymmetric = false;
			}
			return;
		}
		isSymmetric(left.left, right.right);
		isSymmetric(left.right, right.left);
		if (left != null && right != null) {
			if (left.val != right.val) {
				isSymmetric = false;
			}
		}
	}
}

　　非递归代码如下所示：

public class NonLeetCode101 {
	public boolean isSymmetric = true;

	public static class TreeNode {
		int val;
		TreeNode left;
		TreeNode right;

		TreeNode(int x) {
			val = x;
		}
	}

	public boolean isSymmetric(TreeNode root) {
		if (root == null) {
			return true;
		}
		isSymmetric(root.left, root.right);
		return isSymmetric;
	}

	public void isSymmetric(TreeNode left, TreeNode right) {
		if (left == null || right == null) {
			if (left != right) {
				isSymmetric = false;
			}
			return;
		}
		if (left.val != right.val) {
			isSymmetric = false;
			return;
		}
		Stack<TreeNode> lStack = lStruct(left);
		Stack<TreeNode> rStack = rStruct(right);
		if (lStack.size() != rStack.size()) {
			isSymmetric = false;
			return;
		}
		System.out.println("栈大小：" + lStack.size());
		while (lStack.size() > 0) {
			TreeNode n1 = lStack.pop();
			TreeNode n2 = rStack.pop();
			if (n1 == null || n2 == null) {
				if (n1 != n2) {
					isSymmetric = false;
					return;
				}
			} else if (n1 != null && n2 != null) {
				if (n1.val != n2.val) {
					isSymmetric = false;
					return;
				}
			}
		}
	}

	private Stack<TreeNode> lStruct(TreeNode root) {
		Stack<TreeNode> stack = new Stack<>();
		Stack<TreeNode> reStack = new Stack<>();
		stack.push(root);
		while (!stack.isEmpty()) {
			root = stack.pop();
			reStack.push(root);
			if (root != null) {
				if (root.right != null) {
					stack.push(root.right);
				} else if (root.right == null) {
					stack.push(null);
				}
				if (root.left != null) {
					stack.push(root.left);
				} else if (root.left == null) {
					stack.push(null);
				}
			}
		}
		return reStack;
	}

	private Stack<TreeNode> rStruct(TreeNode root) {
		Stack<TreeNode> stack = new Stack<>();
		Stack<TreeNode> reStack = new Stack<>();
		stack.push(root);
		while (!stack.isEmpty()) {
			root = stack.pop();
			reStack.push(root);
			if (root != null) {
				if (root.left != null) {
					stack.push(root.left);
				} else if (root.left == null) {
					stack.push(null);
				}
				if (root.right != null) {
					stack.push(root.right);
				} else if (root.right == null) {
					stack.push(null);
				}
			}
		}
		return reStack;
	}
}

　递归代码如下：

public class LeetCode101 {
	public boolean isSymmetric = true;

	public static class TreeNode {
		int val;
		TreeNode left;
		TreeNode right;

		TreeNode(int x) {
			val = x;
		}
	}

	public boolean isSymmetric(TreeNode root) {
		if (root == null) {
			return true;
		}
		isSymmetric(root.left, root.right);
		return isSymmetric;
	}

	public void isSymmetric(TreeNode left, TreeNode right) {
		if (left == null || right == null) {
			if (left != right) {
				isSymmetric = false;
			}
			return;
		}
		isSymmetric(left.left, right.right);
		isSymmetric(left.right, right.left);
		if (left != null && right != null) {
			if (left.val != right.val) {
				isSymmetric = false;
			}
		}
	}
}