Solution.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} distance
 * @return {number}
 */
var countPairs = function (root, distance) {
    const pairs = [];

    const dfs = node => {
        if (!node) return [];
        if (!node.left && !node.right) return [[node.val, 1]];

        const left = dfs(node.left);
        const right = dfs(node.right);

        for (const [x, dx] of left) {
            for (const [y, dy] of right) {
                if (dx + dy <= distance) {
                    pairs.push([x, y]);
                }
            }
        }

        const res = [];
        for (const arr of [left, right]) {
            for (const x of arr) {
                if (++x[1] <= distance) res.push(x);
            }
        }

        return res;
    };

    dfs(root);

    return pairs.length;
};