Leetcode-1331: array ordinal conversion

subject

Topic linking

Give you an array of integers arr , Please replace each element in the array with their ordinal number after sorting .

The serial number represents how big an element is . The rules for serial numbers are as follows ：

The serial number from 1 Numbered starting .
The bigger an element is , So the bigger the serial number . If two elements are equal , So they have the same serial number .
The serial number of each number should be as small as possible .

Example 1：

 Input ：arr = [40,10,20,30]
 Output ：[4,1,2,3]
 explain ：40  It's the biggest element . 10  It's the smallest element . 20  It's the second smallest number . 30  It's the third smallest number .

Example 2：

 Input ：arr = [100,100,100]
 Output ：[1,1,1]
 explain ： All elements have the same sequence number .

Example 3：

 Input ：arr = [37,12,28,9,100,56,80,5,12]
 Output ：[5,3,4,2,8,6,7,1,3]

Problem solving

Method 1 ： Sort + Hash

Sort , And record the sequence number of each number with hash

class Solution {
    
public:
    vector<int> arrayRankTransform(vector<int>& arr) {
    
        if(arr.size()==0) return {
    };
        vector<int> tmp=arr;
        sort(tmp.begin(),tmp.end());
        unordered_map<int,int> mp;
        int index=1;
        for(int num:tmp){
    
            if(!mp.count(num)) mp[num]=index++;
        }
        vector<int> res;
        for(int num:arr) res.push_back(mp[num]);
        return res;
    }
};