层序遍历
LeetCode 102. Binary Tree Level Order Traversal 二叉树的层序遍历（中等）
LeetCode 103. Binary Tree Zigzag Level Order Traversal 之字形层序遍历
LeetCode 199. Binary Tree Right Side View 找每一层的最右结点
LeetCode 515. Find Largest Value in Each Tree Row 计算每一层的最大值
LeetCode 637. Average of Levels in Binary Tree 计算每一层的平均值
最短路径
LeetCode 1162. As Far from Land as Possible 离开陆地的最远距离（中等）
LeetCode 542. 01 Matrix
LeetCode 994. Rotting Oranges
图结构中求最短路径的问题
LeetCode 310. Minimum Height Trees
