[LeetCode]Binary Search Tree Iterator

第一个解法用了一个HashMap 其实感觉比较笨的方法

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {
    TreeNode cur;
    TreeNode pre;
    HashMap<TreeNode, TreeNode> map = new HashMap<TreeNode, TreeNode>();
    public BSTIterator(TreeNode root) {
        if (root == null) {
            return;
        }
        cur = root;
        while (cur.left != null) {
            map.put(cur, pre);
            pre = cur;
            cur = cur.left;
        }
        map.put(cur, pre);
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return cur != null || pre != null;
    }

    /** @return the next smallest number */
    public int next() {
        int result = cur.val;
        if (cur.right == null) {
            cur = pre;
            pre = map.get(cur);
            map.remove(cur);
        } else {
           cur = cur.right;
           map.put(cur, pre);
           while (cur.left != null) {
               map.put(cur.left, cur);
               cur = cur.left;
           }
           pre = map.get(cur);
           map.remove(cur);
        }
        return result;
    }
}

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = new BSTIterator(root);
 * while (i.hasNext()) v[f()] = i.next();
 */

下面这个用栈的方法明显思路清晰很多了

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {
    Stack<TreeNode> stack = new Stack<TreeNode>();
    public BSTIterator(TreeNode root) {
        while (root != null) {
            stack.push(root);
            root = root.left;
        }
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.isEmpty();
    }

    /** @return the next smallest number */
    public int next() {
        TreeNode node = stack.pop();
        int result = node.val;
        node = node.right;
        while (node != null) {
            stack.push(node);
            node = node.left;
        }
        return result;
    }
}

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = new BSTIterator(root);
 * while (i.hasNext()) v[f()] = i.next();
 */