栈实现二叉树迭代遍历

前序，中序，后序通用

public List<Integer> postorderTraversal(TreeNode root) {
          
   
        List<Integer> res = new ArrayList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        if(root != null){
          
   
            stack.push(root);
        }
        while(!stack.isEmpty()){
          
   
            TreeNode cur = stack.pop();
            if(cur != null){
          
   
            	//这里是后序遍历左，右，根，
            	//由于最后处理根，则需要最先入栈
                stack.push(cur);
                //cur后空结点随之入栈，标识已经访问过，但还没有被处理（还可以进行额外操作）
                stack.push(null);
                //根据前中后顺序的不同，改变cur，left，righ的入栈顺序即可
                if(cur.right != null)
                    stack.push(cur.right);
                if(cur.left != null)
                    stack.push(cur.left);    
            }else{
          
   
                res.add(stack.pop().val);
            }
        }
        return res;
    }

普通迭代

//前序
    public List<Integer> preorderTraversal(TreeNode root) {
          
   
        List<Integer> res = new LinkedList<>();
        if (root == null) return res;
        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()) {
          
   
            if (cur != null) {
          
   
                res.add(cur.val);
                stack.push(cur.right);
                cur = cur.left;
            } else cur = stack.pop();
        }
        return res;
    }

    //中序
    public List<Integer> inorderTraversal(TreeNode root) {
          
   
        Deque<TreeNode> stack = new LinkedList<>();
        List<Integer> list = new LinkedList<>();
        if (root == null) return list;
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()) {
          
   
            if (cur != null) {
          
   
                stack.push(cur);
                cur = cur.left;
            } else {
          
   
                cur = stack.pop();
                list.add(cur.val);
                cur = cur.right;
            }
        }
        return list;
    }

    //后序
    public List<Integer> postorderTraversal(TreeNode root) {
          
   
        List<Integer> res = new LinkedList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode cur = root, pre = null;
        while (cur != null || !stack.isEmpty()) {
          
   
            if (cur != null) {
          
   
                stack.push(cur);
                cur = cur.left;
            } else {
          
   
                cur = stack.peek();
                // 右孩子为空或者访问过了
                if (cur.right == null || cur.right == pre) {
          
   
                    res.add(cur.val);
                    stack.pop();
                    pre = cur;
                    cur = null;
                } else cur = cur.right;
            }
        }
        return res;
    }