[LeetCode] (medium) 519. Random Flip Matrix

https://leetcode.com/problems/random-flip-matrix/

You are given the number of rows n_rows and number of columns n_cols of a 2D binary matrix where all values are initially 0. Write a function flip which chooses a 0 value uniformly at random, changes it to 1, and then returns the position [row.id, col.id] of that value. Also, write a function reset which sets all values back to 0. Try to minimize the number of calls to system's Math.random() and optimize the time and space complexity.

Note:

1. 1 <= n_rows, n_cols <= 10000
2. 0 <= row.id < n_rows and 0 <= col.id < n_cols
3. flip will not be called when the matrix has no 0 values left.
4. the total number of calls to flip and reset will not exceed 1000.

Example 1:

Input: 
["Solution","flip","flip","flip","flip"]
[[2,3],[],[],[],[]]
Output: [null,[0,1],[1,2],[1,0],[1,1]]

Example 2:

Input: 
["Solution","flip","flip","reset","flip"]
[[1,2],[],[],[],[]]
Output: [null,[0,0],[0,1],null,[0,0]]

Explanation of Input Syntax:

The input is two lists: the subroutines called and their arguments. Solution's constructor has two arguments, n_rows and n_cols. flip and resethave no arguments. Arguments are always wrapped with a list, even if there aren't any.

---

 流随机会超时，因为调用太多次rand()

使用逻辑位置与物理位置的重映射

class Solution {
public:
    int row, col;
    int total;
    unordered_map<int, int> M;
    
    Solution(int n_rows, int n_cols) {
        row = n_rows;
        col = n_cols;
        total = row*col;
    }
    
    vector<int> flip() {
        int logic_id = rand() % total;
        int physic_id = logic_id;
        
        if(M.find(logic_id) != M.end()) physic_id = M[logic_id];
        
        M[logic_id] = (M.find(total-1) == M.end()) ? total-1 : M[total-1];
        
        --total;
        
        return {physic_id / col, physic_id % col};
        
    }
    
    void reset() {
        total = row*col;
        M.clear();
    }
};

/**
 * Your Solution object will be instantiated and called as such:
 * Solution* obj = new Solution(n_rows, n_cols);
 * vector<int> param_1 = obj->flip();
 * obj->reset();
 */