Combination sum

Array Backtracking

76.1% acceptance

Jan 12, 2026

20856

532

Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target. You may return the combinations in any order. The same number may be chosen from candidates an unlimited number of times. Two combinations are unique if the frequency of at least one of the chosen numbers is different. The test cases are generated such that the number of unique combinations that sum up to target is less than 150 combinations for the given input.

Solution

Rust

Time O(2^n)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn combination_sum(candidates: Vec<i32>, target: i32) -> Vec<Vec<i32>> {
    let mut result = Vec::new();
    let mut current = Vec::new();
    Self::helper39(&candidates, target, 0, &mut current, &mut result);
    result
  }
  
  fn helper39(
    candidates: &Vec<i32>,
    target: i32,
    start: usize,
    current: &mut Vec<i32>,
    result: &mut Vec<Vec<i32>>,
  ) {
    if target == 0 {
      result.push(current.clone());
      return;
    }
    
    if target < 0 {
      return;
    }
    
    for i in start..candidates.len() {
      current.push(candidates[i]);
      // Can reuse the same element, so pass i instead of i+1
      Self::helper39(candidates, target - candidates[i], i, current, result);
      current.pop();
    }
  }
}