0001.两数之和

方法一：哈希表

时间复杂度 $O(n)$，空间复杂度 $O(n)$。

impl Solution {
    pub fn two_sum(nums: Vec<i32>, target: i32) -> Vec<i32> {
        use std::collections::HashMap;
        let mut map = HashMap::new();
        for (i, num) in nums.iter().enumerate() {
            let complement = target - num;
            if map.contains_key(&complement) {
                return vec![*map.get(&complement).unwrap() as i32, i as i32];
            }
            map.insert(num, i);
        }
        vec![]
    }
}