二分查找有序数组

　　昨天百度面试，问了这样一道题：

　　对于一个有序字符串数组，用二分法查找某一字符串是否存在于该字符串数组中。函数原型为：

bool BinarySearch(const vector<string>& array, const string& target)

　　注意这里的有序指的是字典序，如字符串数组 a, ab, ac, bc, cd, d 就是有序字符串数组，而 a, b, ab 及 a, ac, ab 都不是有序字符串数组。

 

　   对于这道题，一种很笨的做法是：

 1 #include <iostream>
 2 #include <vector>
 3 #include <string>
 4 #include <map>
 5 #include <algorithm>
 6 using namespace std;
 7 
 8 bool BinarySearch(const vector<string>& array, const string& target);
 9 int getUpperBound(const vector<string>& array, const string& target, int index, int low, int high);
10 int getLowerBound(const vector<string>& array, const string& target, int index, int low, int high);
11 
12 int main() 
13 {
14     vector<string> vec{ "ab", "abc", "abc", "abcd", "bcd", "bcde" };
15     string target{ "abcd" };
16     
17     bool ret = BinarySearch(vec, target);
18 
19     return 0;
20 }
21 
22 bool BinarySearch(const vector<string>& array, const string& target)
23 {
24     if (array.size() == 0 && target.length() == 0)
25         return false;
26     else if (array.size() == 0 && target.length() != 0)
27         return false;
28     else if (array.size() != 0 && target.length() == 0)
29     {
30         if (array[0].empty())
31             return true;
32         else
33             return false;
34     }
35     else
36     {
37         int len = target.length();
38         int low = 0, high = array.size() - 1;
39         for (int i = 0; i < len; i++)
40         {
41             int tmpLow = getLowerBound(array, target, i, low, high);
42             int tmpHigh = getUpperBound(array, target, i, low, high);
43             low = tmpLow;
44             high = tmpHigh;
45             if (low == high)
46                 break;
47         }
48     
49         if (array[low] == target)
50             return true;
51     }
52 
53     return false;
54 }
55 
56 int getUpperBound(const vector<string>& array, const string& target, int index, int low, int high)
57 {
58     if (low >= high)
59         return low;
60     
61     while (low < high)
62     {
63         int mid = (low + high) / 2 + 1;
64         if ((index < array[mid].size() && array[mid][index] == target[index]) || index > array[mid].size())
65             low = mid;
66         else
67             high = mid - 1;
68     }
69 
70     return high;
71 }
72 
73 
74 int getLowerBound(const vector<string>& array, const string& target, int index, int low, int high)
75 {
76     if (low >= high)
77         return low;
78 
79     while (low < high)
80     {
81         int mid = (low + high) / 2;
82         if (index < array[mid].size() && array[mid][index] == target[index])
83             high = mid;
84         else if (index > array[mid].size())
85             low = mid + 1;
86         else
87             low = mid + 1;
88     }
89     
90     return low;
91 }

View Code