## 1. Two Sum

Given an array of integers `nums` and an integer `target`, return indices of the two numbers such that they add up to `target`.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

You can return the answer in any order.

Example 1:

```Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Output: Because nums[0] + nums[1] == 9, we return [0, 1].
```

Example 2:

```Input: nums = [3,2,4], target = 6
Output: [1,2]
```

Example 3:

```Input: nums = [3,3], target = 6
Output: [0,1]
```

Constraints:

• `2 <= nums.length <= 103`
• `-109 <= nums[i] <= 109`
• `-109 <= target <= 109`
• Only one valid answer exists.

## Rust Solution

``````struct Solution;

use std::collections::HashMap;

impl Solution {
fn two_sum(nums: Vec<i32>, target: i32) -> Vec<i32> {
let mut hm: HashMap<i32, i32> = HashMap::new();
for (i, &num) in nums.iter().enumerate() {
if let Some(&j) = hm.get(&(target - num)) {
return vec![j, i as i32];
} else {
hm.insert(num, i as i32);
}
}
vec![]
}
}

#[test]
fn test() {
let nums = vec![2, 7, 11, 15];
let target = 9;
assert_eq!(Solution::two_sum(nums, target), vec![0, 1]);
}
``````

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