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;
    }
}