LeetCode 378. Kth Smallest Element in a Sorted Matrix

描述

找出矩阵中的第k小的数

解决

我想到的是使用multiset。

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class Solution {public:    int kthSmallest(vector<vector<int>>& matrix, int k) {        multiset<int> res;        int length = matrix.size();        for (int i = 0; i < length; ++i)        {            for (int j = 0; j < length; ++j)            {                res.insert(matrix[i][j]);            }        }        int cnt = 1, zz = 0;        for (auto it = res.begin(); it != res.end(); ++it, ++cnt)        {            if (k == cnt)            {                zz = *it;                break;            }        }        return zz;    }};

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另一种解法，参考:https://discuss.leetcode.com/topic/54528/clean-solution-with-detailed-explanation-in-c
class Solution {
public:

    int kthSmallest(vector<vector<int>>& matrix, int k) {        int length = matrix.size();        int l = matrix[0][0], r = matrix[length - 1][length - 1];        while (l < r)        {            int m = l + ((r - l) >> 1);            int cnt = 0;            for (int i = 0; i < length; ++i)            {                cnt += upper_bound(matrix[i].begin(), matrix[i].end(), m) - matrix[i].begin();            }            if (cnt < k)            {                l = m + 1;            }            else             {                r = m;            }        }        return l;    }};