带权重的随机选择算法

528. 按权重随机选择

不使用二分法：

class Solution {
private:
    vector<int> preSum;
    int N = 0;
public:
    Solution(vector<int>& w) {
        srand(time(0));
        preSum.push_back(0);
        for (int i = 1; i <= w.size(); i++) {
            preSum.push_back(preSum[i-1] + w[i-1]);
        }
        N = preSum[w.size()];
    }
    
    int pickIndex() {
        int randNum = (rand() % N) + 1;
        
        for (int i = 1; i < preSum.size(); i++) {
            if (randNum > preSum[i-1] && randNum <= preSum[i]) {
                return i-1;
            }
        }
        return -1;
    }
};

使用二分法：

class Solution {
private:
    vector<int> preSum;
    int N = 0;
public:
    Solution(vector<int>& w) {
        preSum.push_back(0);
        for (int i = 1; i <= w.size(); i++) {
            preSum.push_back(preSum[i-1] + w[i-1]);
        }
        N = preSum[w.size()];
    }
    
    int pickIndex() {
        // srand(time(0));
        int randNum = (rand() % N) + 1;
        cout << "randNum:" << randNum << endl;

        int left = 0, right = preSum.size();

        while (left < right) {
            int mid = left + (right - left) / 2;
            if (randNum > preSum[mid-1] && randNum <= preSum[mid]) {
                return mid - 1;
            }
            else if (preSum[mid] < randNum) {
                left = mid + 1;
            }
            else if (preSum[mid] > randNum) {
                right = mid;
            }
        }
        
        return left;
    }
};