0501.二叉搜索树中的众数

/**
 * 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
 * @return {number[]}
 */
var findMode = function (root) {
  const map = new Map();
  const dfs = (node) => {
    if (!node) return;
    map.set(node.val, (map.get(node.val) ?? 0) + 1);
    dfs(node.left);
    dfs(node.right);
  };
  dfs(root);
  let max = 0;
  for (const item of map.values()) {
    if (item > max) {
      max = item;
    }
  }
  const ret = [];
  for (const [key, value] of map.entries()) {
    if (max === value) {
      ret.push(key);
    }
  }
  return ret;
};

/**
 * 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
 * @return {number[]}
 */

var findMode = function (root) {
  let base = 0,
    count = 0,
    maxCount = 0;
  let ret = [];
  const update = (x) => {
    if (x === base) {
      count++;
    } else {
      base = x;
      count = 1;
    }
    if (count === maxCount) {
      ret.push(base);
    }

    if (count > maxCount) {
      maxCount = count;
      ret = [base];
    }
  };

  // Morris 算法：中序遍历
  let cur = root;
  let pre = null;
  while (cur) {
    if (!cur.left) {
      update(cur.val);
      pre = cur;
      cur = cur.right;
      continue;
    }

    pre = cur.left;
    while (pre.right && pre.right !== cur) {
      pre = pre.right;
    }
    if (pre.right) {
      pre.right = null;
      update(cur.val);
      cur = cur.right;
    } else {
      pre.right = cur;
      cur = cur.left;
    }
  }

  return ret;
};