leetcode：1. Two Sum（Java实现）

leetcode测试地址：https://leetcode.com/problems/two-sum/#/description

1. Two Sum

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, and you may not use the same element twice.

Example:

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

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

Subscribe to see which companies asked this question.

package go.jacob.day617;

import java.util.Arrays;
import java.util.HashMap;

/**
* Leetcode:1. Two Sum 注意：输入的数组不是排序的
*
* @author Administrator
*
*/
public class Demo1 {
/*
* 利用hashMap,可以在O(n)时间通过键找到对应的值 键为具体的数，值为下标 牛客网和leetcode下标略有不同
*/
public int[] twoSum(int[] nums, int target) {
int[] result = new int[2];
HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
for (int i = 0; i < nums.length; i++) {
if (map.containsKey(target - nums[i])) {
result[0] = i;
result[1] = map.get(target - nums[i]);
return result;
}
map.put(nums[i], i);
}

return result;
}

/*
* 错误解法：对数组进行了排序。导致数组前后的下标不一致
*/
public int[] twoSum_wrong(int[] nums, int target) {
if (nums == null || nums.length == 0)
return null;
Arrays.sort(nums);
int left = 0;
int right = nums.len
4000
gth - 1;

while (nums[left] + nums[right] != target && left < right) {
if (nums[left] + nums[right] > target)
right--;
else
left++;

}

int[] result = { left, right };
return result;

}
}