Binary Search Tree Iterator (Java)

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.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

不會寫 參考http://blog.csdn.net/whuwangyi/article/details/42304407

Source

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

public class BSTIterator {

	Stack<TreeNode> stack;
	
    public BSTIterator(TreeNode root) {
    	stack = new Stack<TreeNode>();
    	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 root = stack.pop();
        int va = root.val;
        if(root.right != null){
        	root = root.right;
        	while(root != null){
        		stack.push(root);
        		root = root.left;
        	}
        }
        return va;
    }
}

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