出现频度为5：
1. Leet Code OJ 1. Two Sum [Difficulty: Easy]
2. Leet Code OJ 8. String to Integer (atoi) [Difficulty: Easy]
3. Leet Code OJ 15. 3Sum [Difficulty: Medium]
4. Leet Code OJ 20. Valid Parentheses [Difficulty: Easy]
5. Leet Code OJ 21. Merge Two Sorted Lists [Difficulty: Easy]
6. Leet Code OJ 28. Implement strStr() [Difficulty: Easy]
7. Leet Code OJ 56. Merge Intervals [Difficulty: Hard]
8. Leet Code OJ 57. Insert Interval [Difficulty: Hard]
9. Leet Code OJ 65. Valid Number [Difficulty: Hard]
10. Leet Code OJ 70. Climbing Stairs [Difficulty: Easy]
11. Leet Code OJ 73. Set Matrix Zeroes [Difficulty: Medium]
12. Leet Code OJ 88. Merge Sorted Array [Difficulty: Easy]
13. Leet Code OJ 98. Validate Binary Search Tree [Difficulty: Medium]
14. Leet Code OJ 125. Valid Palindrome [Difficulty: Easy]
15. Leet Code OJ 127. Word Ladder [Difficulty: Medium]
出现频度为4：
1. Leet Code OJ 2. Add Two Numbers [Difficulty: Medium]
2. Leet Code OJ 12. Integer to Roman
3. Leet Code OJ 13. Roman to Integer
4. Leet Code OJ 22. Generate Parentheses
5. Leet Code OJ 23. Merge k Sorted Lists
6. Leet Code OJ 24. Swap Nodes in Pairs
7. Leet Code OJ 27. Remove Element [Difficulty: Easy]
8. Leet Code OJ 46. Permutations
9. Leet Code OJ 49. Anagrams
10. Leet Code OJ 67. Add Binary
11. Leet Code OJ 69. Sqrt(x)
12. Leet Code OJ 77. Combinations
13. Leet Code OJ 78. Subsets
14. Leet Code OJ 79. Word Search
15. Leet Code OJ 91. Decode Ways [Difficulty: Medium]
16. Leet Code OJ 102. Binary Tree Level Order Traversal [Difficulty: Easy]
17. Leet Code OJ 129. Sum Root to Leaf Numbers
18. Leet Code OJ 131. Palindrome Partitioning