2014-04-29 00:04
题目：给定一个整数数组，找出所有加起来为指定和的数对。
解法1：可以用哈希表保存数组元素，做到O(n)时间的算法。
代码：
1 // 17.12 Given an array of integers and target value, find all pairs in the array that sum up to the target.
2 // Use hash to achieve O(n) time complexity. Duplicates pairs are skipped.
3 #include <cstdio>
4 #include <unordered_map>
5 #include <vector>
6 using namespace std;
7
8 int main()
9 {
10 vector<int> v;
11 unordered_map<int, int> um;
12 int n, i;
13 int x, y;
14 int target;
15
16 while (scanf("%d", &n) == 1 && n > 0) {
17 scanf("%d", &target);
18
19 v.resize(n);
20 for (i = 0; i < n; ++i) {
21 scanf("%d", &v[i]);
22 }
23
24 // duplicate pairs are skipped
25 for (i = 0; i < n; ++i) {
26 um[v[i]] = um[v[i]] + 1;
27 }
28
29 unordered_map<int, int>::iterator it, it2;
30 for (it = um.begin(); it != um.end(); ++it) {
31 x = it->first;
32 y = target - x;
33 if (x > y) {
34 continue;
35 }
36
37 --it->second;
38 if ((it2 = um.find(y)) != um.end() && it2->second > 0) {
39 printf("(%d, %d)\n", x, y);
40 }
41 ++it->second;
42 }
43
44 v.clear();
45 um.clear();
46 }
47
48 return 0;
49 }
解法2：先将数组排序，然后用两个iterator向中间靠拢进行扫描。总体时间是O(n * log(n))。
代码：
1 // 17.12 Given an array of integers and target value, find all pairs in the array that sum up to the target.
2 // O(n * log(n) + n) solution.
3 #include <algorithm>
4 #include <cstdio>
5 #include <vector>
6 using namespace std;
7
8 int main()
9 {
10 vector<int> v;
11 int n, i;
12 int ll, rr;
13 int target;
14
15 while (scanf("%d", &n) == 1 && n > 0) {
16 scanf("%d", &target);
17
18 v.resize(n);
19 for (i = 0; i < n; ++i) {
20 scanf("%d", &v[i]);
21 }
22 sort(v.begin(), v.end());
23 ll = 0;
24 rr = n - 1;
25
26 int sum;
27 while (ll < rr) {
28 sum = v[ll] + v[rr];
29 if (sum < target) {
30 while (ll + 1 < rr && v[ll] == v[ll + 1]) {
31 ++ll;
32 }
33 ++ll;
34 } else if (sum > target) {
35 while (rr - 1 > ll && v[rr] == v[rr - 1]) {
36 --rr;
37 }
38 --rr;
39 } else {
40 printf("(%d, %d)\n", v[ll], v[rr]);
41 while (ll + 1 < rr && v[ll] == v[ll + 1]) {
42 ++ll;
43 }
44 ++ll;
45 }
46 }
47
48 v.clear();
49 }
50
51 return 0;
52 }
《Cracking the Coding Interview》——第17章：普通题——题目12,码迷,mamicode.com
时间： 2024-10-14 12:11:09