【算法歸納】二叉樹遍歷的非遞歸算法

二叉樹遍歷的常規算法是遞歸，通過一個棧可實現非遞歸算法，許多關於樹的算法題都是這些非遞歸算法的變形，需要熟練掌握。
特別注意後序遍歷的特殊性，在回溯時需要判斷右子樹是否已經訪問過。

1、前序遍歷

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，到達最左下
        if(!stack.isEmpty()){
            cur = stack.pop().right;
        }
    }
}

2、中序遍歷

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、後序遍歷

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;
            }
        }
    }
}