定个小目标，刷个300题

Day 10: 2021-03-27

[51] N-Queens - H
[52] N-Queens II - H

- Since we can only put one Q in one row, so we go thru row by row (loop 1)
- In each row, we go through each cell (potential solution) (loop 2)
- For each cell, check if that cell is valid
- If valid, place Q and continue on the next row (loop 1)
- If not valid, remove Q
- Terminate, if row === n, add the local solution to the results

How to test if it is valid:
- colTaken
- daleTaken (\ diagonal): row + col = constant
- hillTaken (/ diagonal): row - col = constant

Num of Q solved 94 -> 96

Repo: https://github.com/xindixu/leetc ... 8fa1c5175cf8f46c3ce

Day 11: 2021-03-28

Review day

[89] Gray Code - M
[93] Restore IP Addresses - M
[36] Valid Sudoku - M
[37] Sudoku Solver - H
[51] N-Queens - H
[52] N-Queens II - H

Num of Q Reviewed: 6

补一下：
Day 12: 2021-03-29
没刷题。在做公开课的project

补一下：
Day 13: 2021-03-30
刷了一题。在做公开课的project

[80] Remove Duplicates from Sorted Array II - M

Num of Q solved: 96 -> 97

Day 14: 2021-03-31

[33] Search in Rotated Sorted Array - M
[69] Sqrt(x) - E
[34] Find First and Last Position of Element in Sorted Array - M
[81] Search in Rotated Sorted Array II - M

主要focus在binary search
Num of Q solved: 99
Repo: https://github.com/xindixu/leetc ... 839bc7284055cac8048

Day 15: 2021-04-01

[50] Pow(x, n) - M
simple recursion
[162] Find Peak Element - M
binary search, using slope
[209] Minimum Size Subarray Sum - M
two pointers

Num of Q solved: 99 -> 102
Repo: https://github.com/xindixu/leetc ... 692586f171ac4b49b02

Day 16: 2021-04-02

[240] Search a 2D Matrix II - M
Start with the top right corner. R always increment, C always decrement. If cur is too small, increment R. If cur is too big, decrement C.

[387] First Unique Character in a String - E
hash table, loop through it twice

[120] Triangle
DP, reuse original array

[287] Find the Duplicate Number
LinkedList,
index 0 1 5
value 1 5 7
1 -> 5 -> 7
Use index & value to create a linked list. Use tortoise - hare cycle detection to find the entrance of the cycle, which is the duplicated item.
It is the duplicated item because there are multiple indices pointing to that value

Num of Q solved: 102 -> 106
Repo: https://github.com/xindixu/leetc ... 1ca65b520b6693cdef2

Day 17: 2021-04-03
Day 18: 2021-04-04偷懒

Day 18: 2021-04-05

[4] Median of Two Sorted Arrays  - H
想清楚median的definition: a point that divides the array into two halves AND the elements in the left half is smaller than the elements in the right half.

Num of Q solved: 106 -> 107
又是一晚上做一题 好难啊。不过这道题的解法很有意思。

Repo: https://github.com/xindixu/leetc ... eda368bfe407f8e5d6e

Day 19: 2021-04-06

[349] Intersection of Two Arrays - E
[350] Intersection of Two Arrays II - E
[174] Dungeon Game - H
2d DP. Start with what we know -> bottom-right corner
[217] Contains Duplicate - E
[219] Contains Duplicate II

Num of Q solved: 107 -> 112

Day 20: 2021-04-07

Review Day:
[80] Remove Duplicates from Sorted Array II - M
[33] Search in Rotated Sorted Array - M

不会的还是不会，不过稍微看看提示就可以过了。