LeetCode101 SynmetricTree Java题解

题目：

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1   / \  2   2 / \ / \3  4 4  3

But the following is not:

    1   / \  2   2   \   \   3    3

解题：

是否对称就是要求每一层从左到右刚好等于从右到左，有两种解法一种非递归，一种递归。

非递归：

public static boolean isSymmetric(TreeNode root) {if(root==null)return true;Queue<TreeNode> left=new LinkedList<>();Queue<TreeNode> right=new LinkedList<>();if(root.left==null)left.offer(root.left);if(root.right==null)right.offer(root.right);while(!left.isEmpty()&&!right.isEmpty()){TreeNode l=left.poll();TreeNode r=right.poll();if(l==null&&r==null) continue;if(l==null||r==null) return false;if(l.val!=r.val) return false;left.offer(l.left);left.offer(l.right);right.offer(r.right);right.offer(r.left);//倒序插入 形成镜像}if(left.isEmpty()&&right.isEmpty())return true;else {return false;}            }

递归：

 public boolean isSymmetric2(TreeNode root) { if(root==null) return true; else {return  isSame(root.left, root.right);}   }  public boolean isSame(TreeNode left,TreeNode right) { if(left==null&& right==null) return true; if(left==null||right==null) return false; if(left.val!=right.val) return false; else {return isSame(left.left, right.right)&&isSame(left.right, right.left);}  }