Leetcode #1. Two Sum

key： build and search Hash table.

following up questions: Overflow value for target?

用Hash table记录之间的状态，思想类似DP 

def twoSum(nums, target):
    d = {}
    n = len(nums)
    res = []

    for i in range(n):
        if target - nums[i] in d:
            res = [d[target - nums[i]], i]
        else:
            d[nums[i]] = i

    return res