Missing number (Cyclic Sort Sub-pattern)

// Values 1..n (duplicates allowed): put each value at its home index v - 1,
// then every slot that doesn't hold its own value tells you something.
void placeAll(vector<int>& nums) {
    int n = nums.size();
    for (int i = 0; i < n; i++) {
        // keep swapping at i until the value here can't move anywhere useful
        while (nums[nums[i] - 1] != nums[i]) {       // NOT nums[i] != i + 1 (loops on duplicates)
            swap(nums[i], nums[nums[i] - 1]);        // one more value is home for good
        }
    }
}

// After placeAll: slot i holds i + 1, or it holds a squatter.
void readLeftovers(const vector<int>& nums, vector<int>& missing, vector<int>& duplicates) {
    for (int i = 0; i < (int)nums.size(); i++) {
        if (nums[i] != i + 1) {
            missing.push_back(i + 1);                // nobody brought value i + 1
            duplicates.push_back(nums[i]);           // its twin already owns its home
        }
    }
}