# 1. 两数之和

给定一个整数数组 nums 和一个目标值 `target`，请你在该数组中找出和为目标值的那 **两个** 整数，并返回他们的数组下标。

你可以假设每种输入只会对应一个答案。但是，你不能重复利用这个数组中同样的元素。

示例:

> 给定 nums = \[2, 7, 11, 15], target = 9
>
> 因为 nums\[0] + nums\[1] = 2 + 7 = 9 所以返回 \[0, 1]

## 常规暴力解

```javascript
var twoSum = function(nums, target) {
    var i = 0;
    var j = 0;
    for(i = 0; i< nums.length-1; i++){
        for(j= i+1; j < nums.length; j++){
            if(nums[j] === target - nums[i]){
                return [i,j]
            }
        }
    }
};
```

## Map解法

```javascript
var twoSum = function(nums, target) {
    const map = new Map()
    for (let i = 0; i < nums.length; i ++) {
      const otherIndex = map.get(target - nums[i])
      if (otherIndex !== undefined) return [otherIndex, i]
      map.set(nums[i], i)
    }
}
```

## Set有更好的性能

```javascript
const twoSum = (arr, val) => {
  let searchValues = new Set();
  searchValues.add(val - arr[0]);
  for (let i = 1, length = arr.length; i < length; i++) {
    let searchVal = val - arr[i];
    if (searchValues.has(arr[i])) {
      return true;
    } else {
      searchValues.add(searchVal);
    }
  };
  return false;
};
```


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