二叉树的后序遍历

class Solution{
//递归实现
    ArrayList<Integer> list = new ArrayList<Integer>();
    public ArrayList<Integer> postorderTraversal2(TreeNode root) {      
        //后序遍历
        if(root!=null){
        	postorderTraversal2(root.left);
        	postorderTraversal2(root.right);
        	list.add(root.val);
        }
        return list;
    }    
}

class Solution {
	 //循环实现
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();        
        TreeNode cur = root,visit = null;//指向当前节点
        while(cur != null || !stack.isEmpty()){
        		//一直添加左子树
        		if(cur != null){
        			stack.add(cur);
        			cur = cur.left;
        		}else{
        			cur = stack.peek();
        			//右子树被访问过则添加
        			if(cur.right == null || cur.right == visit){
        				cur = stack.pop();
        				list.add(cur.val);
        				visit = cur;
        				cur = null;
        			}//右子树没被访问过
        			else{
        				cur = cur.right;
        			}        		
        		} 
        	}
        
        return list;
    }
}