力扣144，145，94二叉树前、中、后序遍历迭代法统一模板JAVA

public List preorderTraversal(TreeNode root) {//前序
        List result = new ArrayList<>();
        Stack tmp = new Stack<>();
        if(root != null) tmp.push(root);
        TreeNode cur = null;
        while(!tmp.isEmpty()){
            cur = tmp.pop();
            if(cur!=null){
                if(cur.right!=null) tmp.push(cur.right);
                if(cur.left!=null) tmp.push(cur.left);
                tmp.push(cur);
                tmp.push(null); // 中节点访问过，但是还没有处理，加入空节点做为标记。
            } else{// 只有遇到空节点的时候，才将下一个节点放进结果集
                cur = tmp.pop();// 重新取出栈中元素
                result.add(cur.val);
            }
        }
        return result;
    }

public List preorderTraversal(TreeNode root) { //中序
        List result = new ArrayList<>();
        Stack tmp = new Stack<>();
        if(root != null) tmp.push(root);
        TreeNode cur = null;
        while(!tmp.isEmpty()){
            cur = tmp.pop();
            if(cur!=null){
                if(cur.right!=null) tmp.push(cur.right);
                tmp.push(cur);
                tmp.push(null);
                if(cur.left!=null) tmp.push(cur.left);
            }
            else{
                cur = tmp.pop();
                result.add(cur.val);
            }
        }
        return result;
    }

public List preorderTraversal(TreeNode root) { //后序
        List result = new ArrayList<>();
        Stack tmp = new Stack<>();
        if(root != null) tmp.push(root);
        TreeNode cur = null;
        while(!tmp.isEmpty()){
            cur = tmp.pop();
            if(cur!=null){
                tmp.push(cur);
                tmp.push(null);
                if(cur.right!=null) tmp.push(cur.right);
                if(cur.left!=null) tmp.push(cur.left);
            }
            else{
                cur = tmp.pop();
                result.add(cur.val);
            }
        }
        return result;
    }