[LeetCode] questions conclusion_ Binary Search

Binary Search 

T(n) = T(n/2) + O(1)   =>    T(n) = O(lg n)

proof:

 

若是能用iterable , 就用while loop, 能夠防止用recursion的時候stack overflow( process in Linux is 8Mb), stack room is static for each process. （堆空間， heap room 是內存， 是動態的。）層數越多，存儲更多中間/臨時變量，最終超過系統配的stack空間，就會出現stack overflow。

建議：

  　　- 若是問題不復雜，能用iterable就用iterable

　　  - 若是問題比較複雜，仍是用recursive吧

1) ask 是否有duplicates?

　　1.1  若是沒有duplicates

　　標準的binary search:

　　a) l + 1 < r   至關於若是l, r 相鄰就跳出，這樣可以避免死循環. 可是while loop以後須要加一個判斷, 也就是d選項

　　b) l + (r - l)//2      # 不用(l+r)//2 , 由於l + r 有可能overflow，強行裝*，當l + r 之和超過int的最大值，2的32次方減1，會溢出。

　　c) A[mid] ==, <, >    # 爲了bug free, 建議舉個小栗子, 而後畫圖去判斷.

　　d) A[l], A[r] ? target

 

　　Code

　　

def BinarySearch(self, nums, target):  # nums is sorted
    if not nums or target < nums[0] or target > nums[-1]: return -1 l, r = 0, len(nums) -1  # note here is len(nums) -1
    while l + 1 < r: mid = l + (r - l)//2
        if nums[mid] > target: r = mid elif nums[mid] < target: l = mid else: return mid if nums[l] == target: return l if nums[r] == target: return r return -1

 

　　1.2 若是有duplicates

　　a) ask 是first index, last index or dont matter?

　　　　若是是first index:

　　　　　　if A[mid] == target: r = mid

　　　　　　最後判斷 A[l] 和A[r] 的時候先判斷 A[l]

　　　　若是是last index:　　

　　　　　　if A[mid] == target: l = mid

　　　　　　最後判斷 A[l] 和A[r] 的時候先判斷 A[r]

 

 

 

Questions:

[LeetCode] 704. Binary Search_Easy tag: Binary Search

[LeetCode] 374. Guess Number Higher or Lower_Easy tag: Binary Search 　　

[LeetCode] 34. Find First and Last Position of Element in Sorted Array == [LintCode] 61. Search for a Range_Easy tag: Binary Search 　　first index and last index  == target in duplicates. 

[LeetCode] 35. Search Insert Position_Easy tag: Binary Search     first index >= target, without duplicates

[LeetCode] 278. First Bad Version_Easy tag: Binary Search      first index, without duplicates

[LeetCode] 162. Find Peak Element(852. Peak Index in a Mountain Array)_Medium tag: Binary Search   

[LeetCode] 74. Search a 2D Matrix_Medium tag: Binary Search

[LeetCode] 240. Search a 2D Matrix II_Medium tag: Binary Search

 [LeetCode] 69. Sqrt(x)_Easy tag: Binary Search

[LeetCode] 153. Find Minimum in Rotated Sorted Array_Medium tag: Binary Search   first index <= nums[-1]

[LeetCode] 154. Find Minimum in Rotated Sorted Array II_Hard tag: not real binary search anymore

 

[LeetCode] 33. Search in Rotated Sorted Array_Medium tag: Binary Search

[LeetCode] 81. Search in Rotated Sorted Array II_Medium tag: not real binary search anymore

[LeetCode] 702. Search in a Sorted Array of Unknown Size_Medium tag: Binary Search 

[LeetCode] 4. Median of Two Sorted Arrays_Hard tag: Array, Binary Search