第四周题目 Trie树 简单：https://leetcode-cn.com/problems/longest-word-in-dictionary/ 中等：https://leetcode-cn.com/problems/add-and-search-word-data-structure-design/ 困难：https://leetcode-cn.com/problems/word-search-ii/

分治算法 简单：https://leetcode-cn.com/problems/majority-element/ 中等：https://leetcode-cn.com/problems/different-ways-to-add-parentheses/ 困难：https://leetcode-cn.com/problems/burst-balloons/

贪心算法 简单：https://leetcode-cn.com/problems/assign-cookies/ 中等：https://leetcode-cn.com/problems/best-time-to-buy-and-sell-stock-with-transaction-fee/ 困难：https://leetcode-cn.com/problems/ipo/

回溯算法 简单：https://leetcode-cn.com/problems/letter-case-permutation/ 中等：https://leetcode-cn.com/problems/add-and-search-word-data-structure-design/ 困难：https://leetcode-cn.com/problems/n-queens/

动态规划 简单：https://leetcode-cn.com/problems/min-cost-climbing-stairs/ 中等：https://leetcode-cn.com/problems/best-time-to-buy-and-sell-stock-with-cooldown/ 困难：https://leetcode-cn.com/problems/best-time-to-buy-and-sell-stock-iv/ 困难：https://leetcode-cn.com/problems/best-time-to-buy-and-sell-stock-iii/ 困难：https://leetcode-cn.com/problems/edit-distance/