1337. The K Weakest Rows in a Matrix

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# 題目敘述

You are given an m x n binary matrix mat of 1 's (representing soldiers) and 0 's (representing civilians). The soldiers are positioned in front of the civilians. That is, all the 1 's will appear to the left of all the 0 's in each row.

A row i is weaker than a row j if one of the following is true:

• The number of soldiers in row i is less than the number of soldiers in row j .
• Both rows have the same number of soldiers and i < j .

Return the indices of the k weakest rows in the matrix ordered from weakest to strongest.

# 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 in each row is:

• Row 0: 2
• Row 1: 4
• Row 2: 1
• Row 3: 2
• Row 4: 5

The rows ordered from weakest to 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 in each row is:

• Row 0: 1
• Row 1: 4
• Row 2: 1
• Row 3: 1

The rows ordered from weakest to strongest are [0,2,3,1].

# 解題思路

用 sorting 去完成。

# Solution

import java.util.Arrays;

class Solution {

    public int[] kWeakestRows(int[][] mat, int k) {

        int m = mat.length;

        int[][] rowAdd = new int[m][2];

        for(int i = 0; i < m; i++){

            int add = 0;

            for(int num : mat[i]){

                add += num;

            }

            rowAdd[i][0] = add;

            rowAdd[i][1] = i;

        }

        Arrays.sort(rowAdd, (a, b) -> a[0] == b[0] ? a[1] - b[1] : a[0] - b[0]);

        int[] ans = new int[k];

        for(int i = 0; i < k; i++){

            ans[i] = rowAdd[i][1];

        }

        return ans;

    }

}