1. 两数之和

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leading to the advanced question

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方案一: 哈希表

var twoSum = function(nums, target) {
    // 用减法处理就好
    const hashtable = {};
    for (let i = 0; i < nums.length; ++i) {
        if (hashtable[target - nums[i]]  !== undefined) {
            return [i, hashtable[target - nums[i]]]
        }
        hashtable[nums[i]] = i;
    }
};