Leetcode 4.28 Tree Easy

1. 101. Symmetric Tree

　　用递归。

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

     public boolean symmetric(TreeNode p, TreeNode q){
         if(p == null || q == null)
             return p == q? true:false;
         return p.val == q.val && symmetric(p.left, q.right) && symmetric(p.right, q.left);
     }
 }

2. 107. Binary Tree Level Order Traversal II

　　　　list表示当前层的所有节点，nextList表示下一层的所有节点。

 class Solution {
     public List<List<Integer>> levelOrderBottom(TreeNode root) {
         List<List<Integer>> res = new ArrayList();
         List<TreeNode> list = new ArrayList();
         if( root == null)
             return res;
         list.add(root);

         while(!list.isEmpty()){
             List<Integer> curList = new ArrayList();
             List<TreeNode> nextList = new ArrayList();
             for(TreeNode cur: list){   //循环当前层
                 curList.add(cur.val);
                 if(cur.left != null) nextList.add(cur.left);
                 if(cur.right != null) nextList.add(cur.right);
             }
             list = nextList;
             res.add(0, curList);
         }
         return res;
     }
 }