[Leetcode]Binary Search Tree

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.

现将从root到root最左边的element全部加入stack。每次pop的element则为最小。pop后再将pop出的TreeNode的右树最左node全部加入stack。

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class BSTIterator {    public BSTIterator(TreeNode root) {        if(root != null){            stack.push(root);            root = root.left;            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 nextNode = stack.pop();        TreeNode cur = nextNode.right;        while(cur != null){            stack.push(cur);            cur = cur.left;        }        return nextNode.val;    }        private Stack<TreeNode> stack = new Stack<>();}/** * Your BSTIterator will be called like this: * BSTIterator i = new BSTIterator(root); * while (i.hasNext()) v[f()] = i.next(); */