【算法归纳】二叉树遍历的非递归算法

二叉树遍历的常规算法是递归，通过一个栈可实现非递归算法，许多关于树的算法题都是这些非递归算法的变形，需要熟练掌握。
特别注意后序遍历的特殊性，在回溯时需要判断右子树是否已经访问过。

1、前序遍历

public void pre(TreeNode node) {
    Stack<TreeNode> stack = new Stack<>();
    TreeNode cur = node;
    while(cur!=null || !stack.isEmpty()) {
        while(cur!=null){
            System.out.println(cur.val);
            stack.push(cur);
            cur = cur.left;
        }// 至此，到达最左下
        if(!stack.isEmpty()){
            cur = stack.pop().right;
        }
    }
}

2、中序遍历

public void mid(TreeNode node){
    Stack<TreeNode> stack = new Stack<>();
    TreeNode cur = node;
    while(cur!=null || !stack.isEmpty()){
        while(cur!=null){
            stack.push(cur);
            cur=cur.left;
        }
        if(!stack.isEmpty()){
            cur = stack.pop();
            System.out.println(cur.val);
            cur = cur.right;
        }
    }
}

3、后序遍历

public void post(TreeNode node){
    Stack<TreeNode> stack = new Stack<>();
    TreeNode cur = node, record = null;
    while(cur!=null || !stack.isEmpty()){
        while(cur!=null){
            stack.push(cur);
            cur=cur.left;
        }
        if(!stack.isEmpty()) {
            cur = stack.peek();
            if (cur.right != null && cur.right!=record) {
                cur = cur.right;
            } else {
                System.out.println(cur.val);
                stack.pop();
                record = cur;
                cur=null;
            }
        }
    }
}