> For the complete documentation index, see [llms.txt](https://leetcode-solution-leetcode-pp.gitbook.io/leetcode-solution/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://leetcode-solution-leetcode-pp.gitbook.io/leetcode-solution/hard.md).

# 第六章 - 高频考题（困难）

困难难度题目从类型上说多是：

* 图
* 设计题
* 游戏场景题目
* 中等题目的 follow up

从解法上来说，多是：

* 图算法
* 动态规划
* 二分法
* DFS & BFS
* 状态压缩
* 剪枝

从逻辑上说， 要么就是非常难想到，要么就是非常难写代码。 这里我总结了几个技巧：

1. 看题目的数据范围， 看能否暴力模拟
2. 暴力枚举所有可能的算法往上套，比如图的题目。
3. 总结和记忆解题模板，减少解题压力

以下是我列举的经典题目（带 91 字样的表示出自 **91 天学算法**活动）：

* [0004. 寻找两个正序数组的中位数](/leetcode-solution/hard/4.median-of-two-sorted-arrays.md)
* [0023. 合并 K 个升序链表](/leetcode-solution/hard/23.merge-k-sorted-lists.md)
* [0025. K 个一组翻转链表](/leetcode-solution/hard/25.reverse-nodes-in-k-groups.md)
* [0030. 串联所有单词的子串](/leetcode-solution/hard/30.substring-with-concatenation-of-all-words.md)
* [0032. 最长有效括号](/leetcode-solution/hard/32.longest-valid-parentheses.md)
* [0042. 接雨水](/leetcode-solution/hard/42.trapping-rain-water.md)
* [0052. N 皇后 II](/leetcode-solution/hard/52.n-queens-ii.md)
* [0057. 插入区间](https://github.com/azl397985856/leetcode/tree/3ab9bbe39c9b383ac9e532915e5169124d6ab686/collections/problems/57.insert-interval.md)
* [0084. 柱状图中最大的矩形](/leetcode-solution/hard/84.largest-rectangle-in-histogram.md)
* [0085. 最大矩形](/leetcode-solution/hard/85.maximal-rectangle.md)
* [0124. 二叉树中的最大路径和](/leetcode-solution/hard/124.binary-tree-maximum-path-sum.md)
* [0128. 最长连续序列](/leetcode-solution/hard/128.longest-consecutive-sequence.md)
* [0140. 单词拆分 II](https://github.com/azl397985856/leetcode/tree/3ab9bbe39c9b383ac9e532915e5169124d6ab686/collections/problems/140.word-break-ii.md)
* [0145. 二叉树的后序遍历](/leetcode-solution/hard/145.binary-tree-postorder-traversal.md)
* [0212. 单词搜索 II](broken://pages/-MJf1ol6OD7P5Md86Vta)
* [0239. 滑动窗口最大值](/leetcode-solution/hard/239.sliding-window-maximum.md)
* [0295. 数据流的中位数](/leetcode-solution/hard/295.find-median-from-data-stream.md)
* [0297. 二叉树的序列化与反序列化](/leetcode-solution/hard/297.serialize-and-deserialize-binary-tree.md) 91
* [0301. 删除无效的括号](/leetcode-solution/hard/301.remove-invalid-parentheses.md)
* [0312. 戳气球](/leetcode-solution/hard/312.burst-balloons.md)
* [330. 按要求补齐数组](/leetcode-solution/hard/330.patching-array.md)
* [0335. 路径交叉](/leetcode-solution/hard/335.self-crossing.md)
* [0460. LFU 缓存](/leetcode-solution/hard/460.lfu-cache.md)
* [0472. 连接词](/leetcode-solution/hard/472.concatenated-words.md)
* [0488. 祖玛游戏](/leetcode-solution/hard/488.zuma-game.md)
* [0493. 翻转对](/leetcode-solution/hard/493.reverse-pairs.md)
* [0715. Range 模块](/leetcode-solution/hard/715.range-module.md)
* [0768. 最多能完成排序的块 II](/leetcode-solution/hard/768.max-chunks-to-make-sorted-ii.md) 91
* [0887. 鸡蛋掉落](/leetcode-solution/hard/887.super-egg-drop.md)
* [0895. 最大频率栈](/leetcode-solution/hard/895.maximum-frequency-stack.md)
* [0975. 奇偶跳](/leetcode-solution/hard/975.odd-even-jump.md)
* [1032. 字符流](/leetcode-solution/hard/1032.stream-of-characters.md)
* [1168. 水资源分配优化](/leetcode-solution/hard/1168.optimize-water-distribution-in-a-village.md)
* [1203. 项目管理](/leetcode-solution/hard/1203.sort-items-by-groups-respecting-dependencies.md)
* [1255. 得分最高的单词集合](/leetcode-solution/hard/1255.maximum-score-words-formed-by-letters.md)
* [1345. 跳跃游戏 IV](/leetcode-solution/hard/1435.jump-game-iv.md)
* [1449. 数位成本和为目标值的最大数字](/leetcode-solution/hard/1449.form-largest-integer-with-digits-that-add-up-to-target.md)
* [5640. 与数组中元素的最大异或值](/leetcode-solution/hard/5640.maximum-xor-with-an-element-from-array.md)