目录
排序数组(归并排序)
912. 排序数组 - 力扣(LeetCode)https://leetcode.cn/problems/sort-an-array/description/
题解:
class Solution {
public:
vector<int> tmp;
vector<int> sortArray(vector<int>& nums) {
tmp.resize(nums.size());
mergeSort(nums,0,nums.size()-1);//左闭右闭
return nums;
}
void mergeSort(vector<int>& nums,int left,int right)
{
if(left>=right) return;
int mid=left+(right-left)/2;
mergeSort(nums,left,mid); mergeSort(nums,mid+1,right);
int cur1=left,cur2=mid+1,i=0;
while(cur1<=mid && cur2<=right)
tmp[i++]=(nums[cur1]>nums[cur2]?nums[cur2++]:nums[cur1++]);
while(cur1<=mid) tmp[i++]=nums[cur1++];
while(cur2<=right) tmp[i++]=nums[cur2++];
for(int j=left,i=0;j<=right;j++) nums[j]=tmp[i++];
}
};
交易逆序对的总数
LCR 170. 交易逆序对的总数 - 力扣(LeetCode)https://leetcode.cn/problems/shu-zu-zhong-de-ni-xu-dui-lcof/description/
题解:
class Solution {
public:
vector<int> tmp;
int reversePairs(vector<int>& record) {
tmp.resize(record.size());
return mergeSort(record,0,record.size()-1);//左闭右闭
}
int mergeSort(vector<int>& record,int left,int right)
{
if(left>=right) return 0;
int ret=0,mid=left+(right-left)/2;
ret+=mergeSort(record,left,mid);
ret+=mergeSort(record,mid+1,right);
//边合并边计数,排升序
int cur1=left,cur2=mid+1,i=0;
while(cur1<=mid && cur2<=right)
{
if(record[cur1]<=record[cur2])
tmp[i++]=record[cur1++];
else
{
ret+=(mid-cur1+1); tmp[i++]=record[cur2++];
}
}
while(cur1<=mid) tmp[i++]=record[cur1++];
while(cur2<=right) tmp[i++]=record[cur2++];
for(int j=left,i=0;j<=right;j++) record[j]=tmp[i++];
return ret;
}
};
计算右侧小于当前元素的个数
315. 计算右侧小于当前元素的个数 - 力扣(LeetCode)https://leetcode.cn/problems/count-of-smaller-numbers-after-self/description/
题解:
class Solution {
public:
vector<int> ret;
vector<int> tmpIndex;//临时数组的下标
vector<int> index;//原数组元素的下标
vector<int> tmpNums;//临时数组
vector<int> countSmaller(vector<int>& nums) {
int n=nums.size();
tmpIndex.resize(n); tmpNums.resize(n);
index.resize(n); ret.resize(n);
for(int i=0;i<n;i++) index[i]=i;//初始化
mergeSort(nums,0,n-1);//左闭右闭
return ret;
}
void mergeSort(vector<int>& nums,int left,int right)
{
if(left>=right) return;
int mid=left+(right-left)/2;
mergeSort(nums,left,mid); mergeSort(nums,mid+1,right);
int cur1=left,cur2=mid+1,i=0;
while(cur1<=mid && cur2<=right)//排降序
{
if(nums[cur1]<=nums[cur2])//右侧比当前大,只排序
{
tmpNums[i]=nums[cur2];
tmpIndex[i++]=index[cur2++];
}
else//右侧比当前大,计数且排序
{
ret[index[cur1]]+=(right-cur2+1);
tmpNums[i]=nums[cur1];
tmpIndex[i++]=index[cur1++];
}
}
while(cur1<=mid)
{
tmpNums[i]=nums[cur1];
tmpIndex[i++]=index[cur1++];
}
while(cur2<=right)
{
tmpNums[i]=nums[cur2];
tmpIndex[i++]=index[cur2++];
}
//填回原数组
for(int j=left,i=0;j<=right;j++)
{
nums[j]=tmpNums[i]; index[j]=tmpIndex[i++];
}
}
};
翻转对
493. 翻转对 - 力扣(LeetCode)https://leetcode.cn/problems/reverse-pairs/description/
题解:
class Solution {
public:
vector<int> tmp;
int reversePairs(vector<int>& nums) {
int n=nums.size(); tmp.resize(n);
return mergeSort(nums,0,n-1);//左闭右闭
}
int mergeSort(vector<int>& nums,int left,int right)
{
if(left>=right) return 0;
int ret=0,mid=left+(right-left)/2;
ret+=mergeSort(nums,left,mid);
ret+=mergeSort(nums,mid+1,right);
int cur1=left,cur2=mid+1,i=0;
//找翻转对
while(cur1<=mid)
{
while(cur2<=right && nums[cur2]>=nums[cur1]/2.0) cur2++;
if(cur2>right) break;//没有必要遍历了
ret+=(right-cur2+1); cur1++;
}
//合并,排降序
cur1=left,cur2=mid+1,i=0;
while(cur1<=mid && cur2<=right)
tmp[i++]=(nums[cur1]>nums[cur2]?nums[cur1++]:nums[cur2++]);
while(cur1<=mid) tmp[i++]=nums[cur1++];
while(cur2<=right) tmp[i++]=nums[cur2++];
for(int j=left,i=0;j<=right;j++,i++) nums[j]=tmp[i];
return ret;
}
};