Find the kth smallest sum of a matrix with sorted rows

Array Binary Search Heap (Priority Queue) Matrix

62.3% acceptance

Feb 25, 2026

1292

You are given an m x n matrix mat that has its rows sorted in non-decreasing order and an integer k. You are allowed to choose exactly one element from each row to form an array. Return the kth smallest array sum among all possible arrays.

Solution

Rust

Time O(n³)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn kth_smallest(mat: Vec<Vec<i32>>, k: i32) -> i32 {
    let k = k as usize;
    let mut sums: Vec<i32> = mat[0].clone();
    for row in &mat[1..] {
      let mut new_sums = vec![];
      for &s in &sums {
        for &e in row {
          new_sums.push(s + e);
        }
      }
      new_sums.sort_unstable();
      new_sums.truncate(k);
      sums = new_sums;
    }
    sums[k - 1]
  }
}