开始
开始

广度优先

也就是 广度优先算法 bfs Breadth First Search

参考资料

JS 前序遍历、中序遍历、后序遍历、层序遍历详解，深度优先与广度优先区别，附 leetcode 例题题解答案

五三想休息，今天还学习，图解二叉树的层序遍历 BFS（广度优先）模板，附面试题题解

层序遍历

102. 二叉树的层序遍历(中等)

迭代方式解决：

/**
 * 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 levelOrder = function (root) {
  if (root === null) return [];
  const res = [];
  const queue = [root];
  while (queue.length) {
    const len = queue.length;
    const level = [];
    for (let i = 0; i < len; i++) {
      const current = queue.shift();
      level.push(current.val);
      current.left && queue.push(current.left);
      current.right && queue.push(current.right);
    }
    res.push(level);
  }
  return res;
};

递归方式解决：

/**
 * 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 levelOrder = function (root) {
  const res = [];
  function traversal(root, depth) {
    if (root !== null) {
      if (!res[depth]) {
        res[depth] = [];
      }
      res[depth].push(root.val);
      root.left && traversal(root.left, depth + 1);
      root.right && traversal(root.right, depth + 1);
    }
  }
  traversal(root, 0);
  return res;
};

103. 二叉树的锯齿形层序遍历(中等)

/**
 * 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 zigzagLevelOrder = function (root) {
  if (root === null) return [];
  const queue = [root];
  const result = [];
  while (queue.length) {
    const len = queue.length;
    const level = [];
    for (let i = 0; i < len; i++) {
      const current = queue.shift();
      level.push(current.val);
      current.left && queue.push(current.left);
      current.right && queue.push(current.right);
    }
    result.push(level);
  }
  return result.map((i, index) => (index % 2 === 1 ? i.reverse() : i));
};

104. 二叉树的最大深度(简单)

/**
 * 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 maxDepth = function (root) {
  if (!root) return 0;
  if (!root.left && !root.right) return 1;
  let result = 1;
  const queue = [root];
  while (queue.length) {
    // 当前层的节点个数
    const len = queue.length;
    // 逐个让当前层的节点出列
    for (let i = 0; i < len; i++) {
      const current = queue.shift();
      current.left && queue.push(current.left);
      current.right && queue.push(current.right);
    }
    // 当前层所有节点已经出列，如果队列不为空，说明有下一层节点，depth+1
    if (queue.length) {
      result++;
    }
  }
  return result;
};

111. 二叉树的最小深度(简单)

/**
 * 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 minDepth = function (root) {
  if (!root) return 0;
  if (!root.left && !root.right) return 1;
  let result = 1;
  const queue = [root];
  while (queue.length) {
    const len = queue.length;
    for (let index = 0; index < len; index++) {
      const current = queue.shift();
      if (!current.left && !current.right) {
        return result;
      } else {
        current.left && queue.push(current.left);
        current.right && queue.push(current.right);
      }
    }
    result++;
  }
  return result;
};