2-D Dynamic Programming

Overview

When the state needs two indices, usually “position in A × position in B” or “row × column in a grid”, you get a 2-D DP table. The mental moves are the same as 1-D, just on a plane:

• State: dp[i][j] capturing some property of prefixes or a cell.
• Recurrence: usually dp[i][j] depends on dp[i-1][j], dp[i][j-1], or dp[i-1][j-1].
• Space optimization: when dp[i] depends only on dp[i-1], collapse to one row.

Problems

1. 62. Unique Paths (Medium)
2. 1143. Longest Common Subsequence (Medium)
3. 309. Best Time to Buy and Sell Stock with Cooldown (Medium)
4. 518. Coin Change II (Medium)
5. 494. Target Sum (Medium)
6. 97. Interleaving String (Medium)
7. 329. Longest Increasing Path in a Matrix (Hard)
8. 115. Distinct Subsequences (Hard)
9. 72. Edit Distance (Hard)
10. 312. Burst Balloons (Hard)
11. 10. Regular Expression Matching (Hard)

Bonus problems (same pattern, outside NeetCode 150):

• 714. Best Time to Buy and Sell Stock with Transaction Fee (Medium)

Key patterns unlocked here

• Grid path DP: Unique Paths.
• LCS family: Longest Common Subsequence, Edit Distance, Distinct Subsequences, Interleaving String.
• State machine DP: Stock with Cooldown.
• Unbounded knapsack (counting): Coin Change II.
• Sign-partition to subset-sum DP: Target Sum.
• Interval DP: Burst Balloons.
• Regex / pattern DP: Regular Expression Matching.
• Memoized DFS on a matrix: Longest Increasing Path.