501.md

Question:

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.

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).

Analysis

此题有个直观的解法，直接就二叉树遍历，记录每个值出现的次数，组成一个hash，然后遍历一遍这个hash，得知出现的最大次数，由于会出现多个最大的次数，因此，找到了最大的次数之后，还需要再遍历一遍，把出现这个最大次数的值都给找出来，也就是我们的解法一了。

这道题显然没有利用好BST的性质，至于怎么利用，还没有想好。TODO

Tips

Code

// 解法一
func findMode(root *TreeNode) []int {
        var mapping = make(map[int]int)
        findModeCore(root, mapping)

        var maxCount int
        for _, v := range mapping {
                if v > maxCount {
                        maxCount = v
                }
        }

        var ret []int
        for k, v := range mapping {
                if v == maxCount {
                        ret = append(ret, k)
                }
        }
        return ret
}

func findModeCore(root *TreeNode, mapping map[int]int) {
        if root == nil {
                return
        }
        mapping[root.Val]++
        findModeCore(root.Left, mapping)
        findModeCore(root.Right, mapping)
        return
}