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1260. 二维网格迁移

首刷自解

class Solution {
public:
    vector<vector<int>> shiftGrid(vector<vector<int>>& grid, int k) {
        int m = grid.size();
        int n = grid[0].size();
        k %= m * n;
        for (int i = 0; i < k; i++) {
            int tmp = grid[m - 1][n - 1];
            for (int j = m - 1; j >= 0; j--) {
                for (int k = n - 1; k >= 0; k--) {
                    if (j == 0 && k == 0) break;
                    if (k == 0) grid[j][k] = grid[j - 1][n - 1];
                    else grid[j][k] = grid[j][k - 1];
                }
            }
            grid[0][0] = tmp;
        }
        return grid;
    }
};
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算法复杂度：$O(i\ast m\ast n)$， 其中i是k取模m*n
更好的方法是一维展开，算出元素下一个要去的位置

528. 按权重随机选择

首刷自解

class Solution {
public:
    int sums;
    vector<int> prefix;
    Solution(vector<int>& w) {
        sums = 0;
        for (const auto& weight : w) {
            sums += weight;
            prefix.push_back(sums);
        }
    }
    
    int pickIndex() {
        int random = rand() % sums + 1;
        auto it = lower_bound(prefix.begin(), prefix.end(), random);
        return it - prefix.begin();
    }
};

/**
 * Your Solution object will be instantiated and called as such:
 * Solution* obj = new Solution(w);
 * int param_1 = obj->pickIndex();
 */
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算法复杂度：$O(n+k\ast log(n))$，k是调用pickIndex()的次数