075. Sort Colors

 1 class Solution {
 2 public:
 3     void sortColors(vector<int>& nums) {
 4         if (nums.size() == 0) return;
 5         vector<int> start(3, -1);
 6         start[nums[0]] = 0;
 7         for (int i = 1; i < nums.size(); ++i) {
 8             if (nums[i] == nums[i - 1]) continue;
 9             else {
10                 if (start[nums[i]] == -1) {
11                     if (nums[i] == 2) {
12                         start[2] = i; continue;
13                     }
14                     else if (nums[i] == 1) {
15                         if (start[2] == -1) {
16                             start[1] = i; continue;
17                         }
18                         else {
19                             start[1] = start[2];
20                             nums[i] = 2;
21                             nums[start[1]] = 1;
22                             ++start[2];
23                         }
24                     }
25                     else {
26                         // 不可能出现start[1] = start[2] = -1的情况，开始第一个元素已经确定一个
27                         // 这里start[1]和start[2]至少有一个不是-1
28                         if (start[2] != -1) {
29                             nums[start[2]] = 0;
30                             start[0] = start[2];
31                             ++start[2];
32                             nums[i] = 2;
33                             if (start[1] != -1) {
34                                 nums[start[1]] = 0;
35                                 nums[start[0]] = 1;
36                                 start[0] = start[1];
37                                 ++start[1];
38                             }
39                             else continue;
40                         }
41                         else {
42                             nums[start[1]] = 0;
43                             nums[i] = 1;
44                             start[0] = start[1];
45                             ++start[1];
46                         }
47                     }
48                 }
49                 else {
50                     if (nums[i] == nums[i - 1]) continue;
51                     else {
52                         if (nums[i] == 0) {
53                             if (start[2] == -1) {
54                                 nums[i] = 1;
55                                 nums[start[1]] = 0;
56                                 ++start[1];
57                             }
58                             else {
59                                 nums[start[2]] = 0;
60                                 nums[i] = 2;
61                                 ++start[2];
62                                 if (start[1] != -1) {
63                                     nums[start[1]] = 0;
64                                     nums[start[2] - 1] = 1;
65                                     ++start[1];
66                                 }
67                             }
68                         }
69                         else {
70                             // nums[i] == 1, 不可能为2，且前一个一定是2
71                             nums[i] = 2;
72                             nums[start[2]] = 1;
73                             ++start[2];
74                         }
75                     }
76                 }
77             }
78         }
79     }
80 };

 1 class Solution {
 2     public:
 3         void sortColors(vector<int>& A) {
 4             int red = 0, blue = A.size() - 1;
 5             for (int i = 0; i < blue + 1; ) {
 6                 if (A[i] == 0) swap(A[i ++], A[red ++]);
 7                 else if (A[i] == 2) swap(A[i ++], A[blue --]);
 8                 else ++ i;
 9             }
10         }
11 };