Added Combination Sum IV

Author: ryanoneill

README.md

@@ -228,13 +228,13 @@
 - [x] [Missing Number](https://leetcode.com/problems/missing-number/) - [Solution](src/missing_number.rs)
 - [x] [Reverse Bits](https://leetcode.com/problems/reverse-bits/) - [Solution](src/reverse_bits.rs)
 
-### Dynamic Programming
+### Dynamic Programming ✓
 
 - [x] [Climbing Stairs](https://leetcode.com/problems/climbing-stairs/) - [Solution](src/climbing_stairs.rs)
 - [x] [Coin Change](https://leetcode.com/problems/coin-change/) - [Solution](src/coin_change.rs)
 - [x] [Longest Increasing Subsequence](https://leetcode.com/problems/longest-increasing-subsequence/) - [Solution](src/longest_increasing_subsequence.rs)
 - [x] [Word Break](https://leetcode.com/problems/word-break/) - [Solution](src/word_break.rs)
-- [ ] [Combination Sum IV](https://leetcode.com/problems/combination-sum-iv/)
+- [x] [Combination Sum IV](https://leetcode.com/problems/combination-sum-iv/) - [Solution](src/combination_sum_iv.rs)
 - [x] [House Robber](https://leetcode.com/problems/house-robber/) - [Solution](src/house_robber.rs)
 - [x] [House Robber II](https://leetcode.com/problems/house-robber-ii/) - [Solution](src/house_robber_ii.rs)
 - [x] [Decode Ways](https://leetcode.com/problems/decode-ways/) - [Solution](src/decode_ways.rs)

src/combination_sum_iv.rs

@@ -0,0 +1,58 @@
+use std::collections::HashMap;
+
+/// Given an array of *distinct* integers `nums` and a target integer `target`, return the number
+/// of possible combinations that add up to `target`.
+///
+/// The test cases are generated so that the answer can fit in a 32-bit integer.
+struct Solution;
+
+impl Solution {
+
+    fn worker(results: &mut HashMap<i32, i32>, nums: &Vec<i32>, target: i32) -> i32 {
+        let mut result = 0;
+
+        if results.contains_key(&target) {
+            result = results[&target];
+        } else {
+            for &num in nums {
+                let current = target - num;
+                if current == 0 {
+                    result += 1;
+                } else if current > 0 {
+                    result += Self::worker(results, nums, current);
+                }
+            }
+            results.insert(target, result);
+        }
+
+        result
+    }
+
+    pub fn combination_sum4(nums: Vec<i32>, target: i32) -> i32 {
+        let mut results = HashMap::new();
+        Self::worker(&mut results, &nums, target)
+    }
+
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Solution;
+
+    #[test]
+    fn example_1() {
+        let nums = vec![1, 2, 3];
+        let target = 4;
+        let result = Solution::combination_sum4(nums, target);
+        assert_eq!(result, 7);
+    }
+
+    #[test]
+    fn example_2() {
+        let nums = vec![9];
+        let target = 3;
+        let result = Solution::combination_sum4(nums, target);
+        assert_eq!(result, 0);
+    }
+
+}

src/lib.rs

@@ -355,6 +355,7 @@ pub mod sum_of_two_integers; // 371 ✓
 
 pub mod guess_number_higher_or_lower; // 374
 
+pub mod combination_sum_iv; // 377 ✓
 pub mod kth_smallest_element_in_a_sorted_matrix; // 378
 
 pub mod insert_delete_getrandom_o1; // 380 ✓