LeetCode解题报告（295)-- 1337. The K Weakest Rows in a Matrix

Problem

Given a m * n matrix mat of ones (representing soldiers) and zeros (representing civilians), return the indexes of the k weakest rows in the matrix ordered from the weakest to the strongest.

A row i is weaker than row j, if the number of soldiers in row i is less than the number of soldiers in row j, or they have the same number of soldiers but i is less than j. Soldiers are always stand in the frontier of a row, that is, always ones may appear first and then zeros.

Example 1:

Input: mat = 
[[1,1,0,0,0],
 [1,1,1,1,0],
 [1,0,0,0,0],
 [1,1,0,0,0],
 [1,1,1,1,1]], 
k = 3
Output: [2,0,3]
Explanation: 
The number of soldiers for each row is: 
row 0 -> 2 
row 1 -> 4 
row 2 -> 1 
row 3 -> 2 
row 4 -> 5 
Rows ordered from the weakest to the strongest are [2,0,3,1,4]

Example 2:

Input: mat = 
[[1,0,0,0],
 [1,1,1,1],
 [1,0,0,0],
 [1,0,0,0]], 
k = 2
Output: [0,2]
Explanation: 
The number of soldiers for each row is: 
row 0 -> 1 
row 1 -> 4 
row 2 -> 1 
row 3 -> 1 
Rows ordered from the weakest to the strongest are [0,2,3,1]

Constraints:

• m == mat.length
• n == mat[i].length
• 2 <= n, m <= 100
• 1 <= k <= m
• matrix[i][j] is either 0 or 1.

---

Analysis

  题目给出一个矩阵，我们需要对行进行排序，排序的规则是，行里面1的数量多的靠后，数量少的靠前，最后需要返回行的坐标。解法也比较常规，直接统计每一行1的数量，然后把这个数量和该行对应的下标组成一个pair，放到一个数组中。统计完成后对数组进行排序，最后返回前k个即可。

---

Solution

  在构成pair的时候，需要注意是1的数量放在第一个元素，下标放在第二个元素，这样使用sort()的时候默认是对第一个元素进行排序。

---

Code

class Solution {
public:
    vector<int> kWeakestRows(vector<vector<int>>& mat, int k) {
        vector<vector<int>> rows;
        int m = mat.size();
        int n = mat[0].size();
        
        for (int i = 0; i < m; i++) {
            int sum = 0;
            for (int j = 0; j < n; j++) {
                if (mat[i][j] == 1) {
                    sum++;
                }
            }
            rows.push_back({sum, i});
        }
        
        sort(rows.begin(), rows.end());
        vector<int> result;
        for (int i = 0; i < k; i++) {
            result.push_back(rows[i][1]);
        }
        return result;
    }
};

---

Summary

  这道题目的分享到这里，谢谢您的支持！