【LeetCode】二叉搜索树的前序，中序，后续遍历非递归方法

前序遍历

public List<Integer> preorderTraversal(TreeNode root) {
       ArrayList<Integer> list = new ArrayList<Integer>();
       Stack<TreeNode> stack=new Stack<TreeNode>();
       TreeNode p=root;
       while(!stack.isEmpty()||p!=null){
           if(p!=null){
               stack.push(p);
               list.add(p.val); // Add before going to children
               p=p.left;
           }else{
               TreeNode node=stack.pop();
               p=node.right;
           }
       }
       return list;
   }　　

中序遍历

public List<Integer> inorderTraversal(TreeNode root) {
       ArrayList<Integer> list = new ArrayList<Integer>();
       Stack<TreeNode> stack=new Stack<TreeNode>();
       TreeNode p=root;
       while(!stack.isEmpty()||p!=null){
           if(p!=null){
               stack.push(p);
               p=p.left;
           }else{
               TreeNode node=stack.pop();
               list.add(node.val);// Add after all left children
               p=node.right;
           }
       }
       return list;
   }

后序遍历

public List<Integer> postorderTraversal(TreeNode root) {
        LinkedList<Integer> list = new LinkedList<Integer>();
        Stack<TreeNode> stack=new Stack<TreeNode>();
        TreeNode p=root;
        while(!stack.isEmpty()||p!=null){
            if(p!=null){
                stack.push(p);
                list.addFirst(p.val);// Reverse the process of preorder
                p=p.right;
            }else{
                TreeNode node=stack.pop();
                p=node.left;
            }
        }
        return list;
    }