173. Binary Search Tree Iterator - jiejackyzhang/leetcode-note GitHub Wiki

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.

Tree类题目。

采用stack。先将沿root的所有左孩子存入stack。 每次调用next()，将栈顶元素pop出来，然后以它的右孩子为新的root。

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

public class BSTIterator {
    
    private Stack<TreeNode> stack;

    public BSTIterator(TreeNode root) {
        stack = new Stack<TreeNode>();
        pushAllLeft(root);
    }

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

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

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