315. Count of Smaller Numbers After Self(二分或者算法导论中的归并求逆序数对)
You are given an integer array nums and you have to return a new counts array. The counts array has the property where counts[i]
is the number of smaller elements to the right of nums[i]
.
Example 1:
Input: nums = [5,2,6,1]
Output: [2,1,1,0]
Explanation:
To the right of 5 there are 2 smaller elements (2 and 1).
To the right of 2 there is only 1 smaller element (1).
To the right of 6 there is 1 smaller element (1).
To the right of 1 there is 0 smaller element.
Constraints:
0 <= nums.length <= 10^5
-10^4 <= nums[i] <= 10^4
class Solution {
public:
vector<int> countSmaller(vector<int>& nums) {
vector<int> res(nums.size(),0);
//从右向左,将数组有序插入tmp.利用二分查找确定当前数右边比它小的数的个数
vector<int> tmp;
for(int i=nums.size()-1;i>=0;i--){
int left = 0,right=tmp.size()-1;
//找第一个大于等于当前数的位置。插入其中
while(left <= right){
int mid = left+(right-left)/2;
if(tmp[mid] < nums[i]) left = mid+1;
else right = mid-1;
}
//最后返回的位置是left
res[i]=left;
//插入nums[i]
tmp.insert(tmp.begin()+left,nums[i]);
}
return res;
}
};
//归并:先引入逆序数;不同于逆序数对:
res[nums[i].second] += (j-mid-1);
这个里面坑比较多
class Solution {
public:
//法二:利用归并排序求逆序对数的方法
//https://leetcode-cn.com/problems/shu-zu-zhong-de-ni-xu-dui-lcof/submissions/ vector<int> countSmaller(vector<int>& nums) {
int n=nums.size();
vector<int> res(n,0);
if(n==0 || n==1) return res;
vector<pair<int,int>> tmp(n,pair<int,int>{0,0});
vector<pair<int,int>> idx;
for(int i=0;i<n;i++){
idx.push_back(make_pair(nums[i],i));
}
mergesort(idx,tmp,0,n-1,res);
return res;
} //merge的过程将left到right有序重新存入nums.归并nums[left,mid],nums[mid+1,right]
void merge(vector<pair<int,int>>& nums,vector<pair<int,int>>& tmp,int left,int mid,int right,vector<int>& res) {
int i=left,j=mid+1,k=left;
for(;i<=mid&&j<=right;){
if(nums[i].first<=nums[j].first){
//不同于算整个数组逆序数
//这里的i不是之前的i。归并后数字的位置被改变了.所以利用pari记录nums[i]原始位置
//res[i] += (j-mid-1);
res[nums[i].second] += (j-mid-1);
tmp[k++] = nums[i++];
}else{
tmp[k++] = nums[j++];
}
}
//还有未归并完成的
while(i<=mid){
//先计算res
res[nums[i].second] += (j-mid-1);
tmp[k++]=nums[i++];
}
while(j<=right){
tmp[k++]=nums[j++];
}
//将tmp重新放入nums,那么nums[left,right]即有序了
for(int i=left;i<=right;i++){
nums[i] = tmp[i];
}
return;
}
//归并排序
void mergesort(vector<pair<int,int>>& nums,vector<pair<int,int>>& tmp,int left,int right,vector<int>& res) {
if(left < right){
int mid = left+(right-left)/2;
mergesort(nums,tmp,left,mid,res);
mergesort(nums,tmp,mid+1,right,res);
//合并nums[left,mid] nums[mid+1,right]
merge(nums,tmp,left,mid,right,res);
}
return;
} };
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