1. Two Sum

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#Array #Hash Table #Leetcode #Two Sum

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本文介绍了解决LeetCode上经典题目“两数之和”的方法。通过使用哈希表，该算法能在O(n)的时间复杂度内找到数组中两个数相加等于目标值的下标。

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Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

UPDATE (2016/2/13):
The return format had been changed to zero-based indices. Please read the above updated description carefully.

Leetcode第一题，用hashmap 可简单解。遍历数组，查找hashmap 里有没有target - nums[i], 有则返回位置，没有则把当前nums[i] 存入hashmap。代码如下：

public class Solution {
    public int[] twoSum(int[] nums, int target) {
        HashMap<Integer,Integer> ht = new HashMap<Integer,Integer>();
        int[] result = new int[2];
        ht.put(nums[0], 0);
        for (int i = 1; i < nums.length; i++) {
            if (ht.containsKey(target - nums[i])) {
                result[0] = ht.get(target - nums[i]);
                result[1] = i;
            } else {
                ht.put(nums[i], i);
            }
        }
        return result;
    }
}