| 3 | Longest Substring Without Repeating Characters | — | Medium | Hash TableStringSliding Window | 37% | |
| 5 | Longest Palindromic Substring | — | Medium | Two PointersStringDynamic Programming | 36% | |
| 6 | Zigzag Conversion | — | Medium | String | 51% | |
| 8 | String to Integer (atoi) | — | Medium | String | 19% | |
| 10 | Regular Expression Matching | — | Hard | StringDynamic ProgrammingRecursion | 29% | |
| 12 | Integer to Roman | — | Medium | Hash TableMathString | 68% | |
| 13 | Roman to Integer | — | Easy | Hash TableMathString | 65% | |
| 14 | Longest Common Prefix | — | Easy | StringTrie | 45% | |
| 17 | Letter Combinations of a Phone Number | — | Medium | Hash TableStringBacktracking | 64% | |
| 20 | Valid Parentheses | — | Easy | StringStack | 42% | |
| 22 | Generate Parentheses | — | Medium | StringDynamic ProgrammingBacktracking | 77% | |
| 28 | Find the Index of the First Occurrence in a String | — | Easy | Two PointersStringString Matching | 45% | |
| 30 | Substring with Concatenation of All Words | — | Hard | Hash TableStringSliding Window | 33% | |
| 32 | Longest Valid Parentheses | — | Hard | StringDynamic ProgrammingStack | 36% | |
| 38 | Count and Say | — | Medium | String | 60% | |
| 43 | Multiply Strings | — | Medium | MathStringSimulation | 42% | |
| 44 | Wildcard Matching | — | Hard | StringDynamic ProgrammingGreedy | 30% | |
| 49 | Group Anagrams | — | Medium | ArrayHash TableString | 71% | |
| 58 | Length of Last Word | — | Easy | String | 56% | |
| 65 | Valid Number | — | Hard | String | 21% | |
| 67 | Add Binary | — | Easy | MathStringBit Manipulation | 56% | |
| 68 | Text Justification | — | Hard | ArrayStringSimulation | 48% | |
| 71 | Simplify Path | — | Medium | StringStack | 47% | |
| 72 | Edit Distance | — | Medium | StringDynamic Programming | 59% | |
| 76 | Minimum Window Substring | — | Hard | Hash TableStringSliding Window | 45% | |
| 79 | Word Search | — | Medium | ArrayStringBacktracking | 45% | |
| 87 | Scramble String | — | Hard | StringDynamic Programming | 42% | |
| 91 | Decode Ways | — | Medium | StringDynamic Programming | 36% | |
| 93 | Restore IP Addresses | — | Medium | StringBacktracking | 53% | |
| 97 | Interleaving String | — | Medium | StringDynamic Programming | 42% | |
| 115 | Distinct Subsequences | — | Hard | StringDynamic Programming | 50% | |
| 125 | Valid Palindrome | — | Easy | Two PointersString | 51% | |
| 126 | Word Ladder II | — | Hard | Hash TableStringBacktracking | 27% | |
| 127 | Word Ladder | — | Hard | Hash TableStringBreadth-First Search | 42% | |
| 131 | Palindrome Partitioning | — | Medium | StringDynamic ProgrammingBacktracking | 72% | |
| 132 | Palindrome Partitioning II | — | Hard | StringDynamic Programming | 35% | |
| 139 | Word Break | — | Medium | ArrayHash TableString | 48% | |
| 140 | Word Break II | — | Hard | ArrayHash TableString | 53% | |
| 151 | Reverse Words in a String | — | Medium | Two PointersString | 51% | |
| 165 | Compare Version Numbers | — | Medium | Two PointersString | 42% | |
| 166 | Fraction to Recurring Decimal | — | Medium | Hash TableMathString | 26% | |
| 168 | Excel Sheet Column Title | — | Easy | MathString | 43% | |
| 171 | Excel Sheet Column Number | — | Easy | MathString | 66% | |
| 179 | Largest Number | — | Medium | ArrayStringGreedy | 41% | |
| 187 | Repeated DNA Sequences | — | Medium | Hash TableStringBit Manipulation | 51% | |
| 205 | Isomorphic Strings | — | Easy | Hash TableString | 47% | |
| 208 | Implement Trie (Prefix Tree) | — | Medium | Hash TableStringDesign | 68% | |
| 211 | Design Add and Search Words Data Structure | — | Medium | StringDepth-First SearchDesign | 47% | |
| 212 | Word Search II | — | Hard | ArrayStringBacktracking | 37% | |
| 214 | Shortest Palindrome | — | Hard | StringRolling HashString Matching | 41% | |