173.二叉搜索树迭代器

本题就是实现二叉树的中序遍历，利用数组本身实现迭代器

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class BSTIterator {
    private int idx;
    private List<Integer> arr;

    public BSTIterator(TreeNode root) {
        idx = 0;
        arr = new ArrayList<>();
        inorderTravel(root,arr);
    }
    
    public int next() {
        return arr.get(idx++);
    }
    
    public boolean hasNext() {
        return idx < arr.size();
    }

    //中序遍历
    public void inorderTravel(TreeNode root,List<Integer> arr){
        if(root == null){
            return;
        }
        inorderTravel(root.left,arr);
        arr.add(root.val);
        inorderTravel(root.right,arr);
    }
}

/**
 * Your BSTIterator object will be instantiated and called as such:
 * BSTIterator obj = new BSTIterator(root);
 * int param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */