Lintcode 11. Search Range in Binary Search Tree

Given a binary search tree and a range [k1, k2], return node values within a given range in ascending order.

Example

Example 1:

Input：{5},6,10
Output：[]
        5
it will be serialized {5}
No number between 6 and 10

Example 2:

Input：{20,8,22,4,12},10,22
Output：[12,20,22]
Explanation：
        20
       /  \
      8   22
     / \
    4   12
it will be serialized {20,8,22,4,12}
[12,20,22] between 10 and 22

Code (Java):

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: param root: The root of the binary search tree
     * @param k1: An integer
     * @param k2: An integer
     * @return: return: Return all keys that k1<=key<=k2 in ascending order
     */
    public List<Integer> searchRange(TreeNode root, int k1, int k2) {
        // write your code here
        List<Integer> ans = new ArrayList<>();
        if (root == null) {
            return ans;
        }
        
        // stack stores all nodes greater or equal to k1
        //
        Stack<TreeNode> stack = new Stack<>();
        
        TreeNode p = root;
        
        while(p != null) {
            if (p.val >= k1) {
                stack.push(p);
                p = p.left;
            } else {
                p = p.right;
            }
        }
        
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node.val <= k2) {
                ans.add(node.val);
            }
            
            findNext(node.right, stack);
        }
        
        return ans;
    }
    
    private void findNext(TreeNode p, Stack<TreeNode> stack) {
        while (p != null) {
            stack.push(p);
            p = p.left;
        }
    }
}