[x] Flatten List

[x] Flatten Nested List Iterator

[x] Flatten 2D Vector

[x] Zigzag Iterator

[x] Zigzag Iterator II

[x] Flatten Binary Tree to Linked List

[x] Subarray Sum

[x] Subarray Sum II

[x] Submatrix Sum

[x] Continuous Subarray Sum

[x] Continuous Subarray Sum II

[x] Subarray Sum Closest

[x] Minimum Size Subarray Sum

[ ] Kth Smallest Number in Sorted Matrix

[ ] Find Peak Element

[ ] Find Peak Element II

[ ] Trapping water I

[ ] Trapping water II

[x] Sliding Window Matrix Maximum

[ ] Number of airplanes in the sky

[x] Meeting room

[x] Maximum Gap

[ ] Nested List Weight Sum

[ ] Bomb Enemy

[ ] Build Post Office

[ ] Build Post Office II