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Approach: Traverse counting value frequencies, then return all keys with the maximum count.

Complexity: O(n) time, O(n) space

js

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var findMode = function(root) {
    const map = {}
    traverse(root, map)
    const maxCount = Math.max(...Object.values(map))
    return Object.keys(map).reduce((ret, key) => {
        if (map[key] === maxCount) {
            ret.push(parseInt(key))
        }
        return ret
    }, [])
};

function traverse(root, map) {
    if (root === null) {
        return
    }
    map[root.val] = (map[root.val] || 0) + 1
    traverse(root.left, map)
    traverse(root.right, map)
}