860. 柠檬水找零

 1 class Solution {
 2 public:
 3     bool lemonadeChange(vector<int>& bills) {
 4         int five = 0;
 5         int ten = 0;
 6         int twenty = 0;
 7         for(int i = 0; i < bills.size(); i++){
 8             switch(bills[i]){
 9                 case 5:
10                     five++;
11                     break;
12                 case 10:
13                     if(five > 0){
14                         five--;
15                         ten++;
16                     }else{
17                         return false;
18                     }
19                     break;
20                 case 20:
21                     if(five == 0){
22                         return false;
23                     }else{
24                         if(ten > 0){
25                             ten--;
26                             five--;
27                             twenty++;
28                         }else{
29                             if(five >= 3){
30                                 five -= 3;
31                                 twenty++;
32                             }else{
33                                 return false;
34                             }
35                         }
36                     }
37                     break;
38             }
39         }
40         return true;
41     }
42 };

406. 根据身高重建队列

先按照身高排序，身高高且位置小的排前面，然后遍历数组，按位置顺序插入新数组

 1 class Solution {
 2 public:
 3     static bool comp(const vector<int>& a, const vector<int>& b){
 4         if(a[0] == b[0])    return a[1] < b[1];
 5         return a[0] > b[0];
 6     }
 7     vector<vector<int>> reconstructQueue(vector<vector<int>>& people) {
 8         sort(people.begin(),people.end(),comp);
 9         vector<vector<int>> que;
10         for(int i = 0; i < people.size(); i++){
11             int pos = people[i][1];
12             que.insert(que.begin() + pos, people[i]);
13         }
14         return que;
15     }
16 };

452. 用最少数量的箭引爆气球

 1 class Solution {
 2 public:
 3     static bool comp(const vector<int>& a, const vector<int>& b){
 4         return a[0] < b[0];
 5     }
 6     int findMinArrowShots(vector<vector<int>>& points) {
 7         if(points.size() == 0)  return 0;
 8         sort(points.begin(),points.end(),comp);
 9         int result = 1;
10         for(int i = 1; i < points.size(); i++){
11             if(points[i][0] > points[i-1][1])// 跟前一个不挨着
12                 result++;
13             else{
14                 // 更新重叠气球，把这俩看作一个
15                 points[i][1] = min(points[i][1],points[i-1][1]);
16             }
17         }
18         return result;
19     }
20 };