501. Find Mode in Binary Search Tree 找二叉搜索树的众数

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).

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int[] FindMode(TreeNode root) {
		List<int> modes = new List<int>();
		int modeCount = 0, lastValue = int.MinValue, lastValueCount = 0;
		Stack<TreeNode> stack = new Stack<TreeNode>();
		TreeNode current = root;
		while (current != null) {
			stack.Push(current);
			current = current.left;
		}
		while (stack.Count != 0) {
			current = stack.Pop();
			if (current.val == lastValue) {
				lastValueCount++;
			} else {
				lastValue = current.val;
				lastValueCount = 1;
			}
			if (lastValueCount > modeCount) {
				modes.Clear();
				modes.Add(current.val);
				modeCount = lastValueCount;
			} else if (lastValueCount == modeCount) {
				modes.Add(current.val);
			}
			TreeNode node = current.right;
			while (node != null) {
				stack.Push(node);
				node = node.left;
			}
		}
		return modes.ToArray();
    }
}

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