173 Binary Search Tree Iterator

173 Binary Search Tree Iterator

Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

Example:

BSTIterator iterator = new BSTIterator(root);
iterator.next();    // return 3
iterator.next();    // return 7
iterator.hasNext(); // return true
iterator.next();    // return 9
iterator.hasNext(); // return true
iterator.next();    // return 15
iterator.hasNext(); // return true
iterator.next();    // return 20
iterator.hasNext(); // return false

Note:

• next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.
• You may assume that next() call will always be valid, that is, there will be at least a next smallest number in the BST when next() is called.

题目大意

构造二叉排序树，实现next和hasnext函数，next函数返回下一个中序遍历的数。

思路

非递归中序遍历二叉树就是用栈来实现的，先一直向左遍历，遍历过程中将节点压栈，直到左孩子为空，再出栈，并对其右孩子同样操作。

代码

/**
 * 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 {
    Stack<TreeNode> ans;
    public BSTIterator(TreeNode root) {
        this.ans = new Stack<TreeNode>();
        this.leftmosttravel(root);
    }
    public void leftmosttravel(TreeNode root){
        while(root != null){
            this.ans.push(root);
            root = root.left;
        }
    }
    /** @return the next smallest number */
    public int next() {
        TreeNode t = this.ans.pop();
        if(t.right != null){
            this.leftmosttravel(t.right);
        }
        return t.val;
    }
    
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return this.ans.size() > 0;
    }
}

/**
 * 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();
 */