[数据结构与算法]二叉搜索树中第K小的元素

二叉搜索树中第K小的元素

/**
 * 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 Solution {
    public int kthSmallest(TreeNode root, int k) {
        //中序遍历

        //栈
        Deque<TreeNode> stack = new ArrayDeque<TreeNode>();

        while (root != null || !stack.isEmpty()) {
            //把左边的存起来，也就是优先把更小的压到栈底
            while (root != null) {
                stack.push(root);
                root = root.left;
            }

            //出栈开始计数
            root = stack.pop();
            k--;

            //要的就是找到第k小的数
            if (k == 0)
                break;

            root = root.right;
        }
        return root.val;
    }
}