add: 二叉树的层序遍历

src/tree/binary-tree-level-order-traversal.js

@@ -0,0 +1,28 @@
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {number[][]}
+ */
+export const levelOrder = function (root) {
+  const res = []
+  const queue = [root]
+  while (queue.length) { // BFS
+    const tmp = []
+    const leave = queue.length // 记录这一层有几个
+    for (let i = 0; i < leave; i++) { // 一次性把固定个数的队列执行完
+      const node = queue.shift()
+      if (node.left) queue.push(node.left)
+      if (node.right) queue.push(node.right)
+      tmp.push(node.val)
+    }
+    res.push(tmp)
+  }
+
+  return res
+}