General approach for problem patterns

[wuyichen24]

• Combinations and Permutations: https://leetcode.com/problems/subsets/solutions/27281/a-general-approach-to-backtracking-questions-in-java-subsets-permutations-combination-sum-palindrome-partitioning/
• Basic calculators: https://leetcode.com/problems/basic-calculator-iii/solutions/113592/development-of-a-generic-solution-for-the-series-of-the-calculator-problems/
• From good to great. How to approach most of DP problems: https://leetcode.com/problems/house-robber/solutions/156523/from-good-to-great-how-to-approach-most-of-dp-problems/
• Here is a 10-line template that can solve most 'substring' problems: https://leetcode.com/problems/minimum-window-substring/solutions/26808/here-is-a-10-line-template-that-can-solve-most-substring-problems/