1051. Height Checker

A school is trying to take an annual photo of all the students. The students are asked to stand in a single file line in non-decreasing order by height. Let this ordering be represented by the integer array expected where expected[i] is the expected height of the ith student in line.

You are given an integer array heights representing the current order that the students are standing in. Each heights[i] is the height of the ith student in line (0-indexed).

Return the number of indices where heights[i] != expected[i].

class Solution {
public:
    int heightChecker(vector<int>& heights) {
        int res = 0;
        unordered_map<int, int> counter;
        for(auto n : heights) {
            counter[n]++;
        }
        for(int i = 1, j = 0; i <= 100 and j < heights.size(); j++) {
            while(i < 100 and (!counter.count(i) or counter[i] == 0)) i++;
            if(i == heights[j]) res++;
            counter[i]--;
        }
        return heights.size() - res;
    }
};