LeetCode_501. Find Mode in Binary Search Tree

Posted 2020-11-25 邓戈麟

 

501. Find Mode in Binary Search Tree

Easy

Given a binary search tree (BST) with duplicates, find all the mode(s) (the most frequently occurred element) in the given BST.

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than or equal to the node‘s key.
• The right subtree of a node contains only nodes with keys greater than or equal to the node‘s key.
• Both the left and right subtrees must also be binary search trees.

 

For example:
Given BST [1,null,2,2],

   1
         2
    /
   2

 

return [2].

Note: If a tree has more than one mode, you can return them in any order.

Follow up: Could you do that without using any extra space? (Assume that the implicit stack space incurred due to recursion does not count).

 

package leetcode.easy;

/**
 * Definition for a binary tree node. public class TreeNode { int val; TreeNode
 * left; TreeNode right; TreeNode(int x) { val = x; } }
 */
public class FindModeInBinarySearchTree {
	private static void print_arr(int[] nums) {
		for (int num : nums) {
			System.out.print(num + " ");
		}
		System.out.println();
	}

	java.util.Map<Integer, Integer> map;
	int max = 0;

	public int[] findMode(TreeNode root) {
		this.map = new java.util.HashMap<>();
		inorder(root);
		java.util.List<Integer> list = new java.util.LinkedList<>();
		for (int key : map.keySet()) {
			if (map.get(key) == max) {
				list.add(key);
			}
		}
		int[] num = new int[list.size()];
		for (int i = 0; i < list.size(); i++) {
			num[i] = list.get(i);
		}
		return num;
	}

	public void inorder(TreeNode root) {
		if (root == null) {
			return;
		}
		map.put(root.val, map.getOrDefault(root.val, 0) + 1);
		if (map.get(root.val) > max) {
			max = map.get(root.val);
		}
		inorder(root.left);
		inorder(root.right);
	}

	@org.junit.Test
	public void test() {
		TreeNode tn11 = new TreeNode(1);
		TreeNode tn22 = new TreeNode(2);
		TreeNode tn33 = new TreeNode(2);
		tn11.left = null;
		tn11.right = tn22;
		tn22.left = tn33;
		tn22.right = null;
		tn33.left = null;
		tn33.right = null;
		print_arr(findMode(tn11));
	}
}