LeetCode | Two Sum
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Given an array of integers, find two numbers such that they add up to a specific target number.
The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.
You may assume that each input would have exactly one solution.
Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2
两种破法:
1.先排序,然后从两头向中间一步一步查找。时间复杂度为排序的时间复杂度,用快排可以达到O(N*lgN);
2.利用HashMap,时间复杂度O(n)。
我比较懒,直接用第二种方法。
以为会一次运行,马上accept,有个地方要注意(红色标记处),即排除target=6,有个元素是3的情况,map找的是本身。解决之后AC。
public class Solution {
public int[] twoSum(int[] numbers, int target) {
HashMap<Integer,Integer> map = new HashMap<Integer,Integer>(numbers.length);
int[] result = new int[2];
for(int i=0; i<numbers.length; i++) {
Integer temp = numbers[i];
map.put(temp, i);
}
for(int i=0; i<numbers.length; i++) {
Integer a = map.get(target - numbers[i]);
if(a != null && a!=i){
result[0] = i < a ? i+1 : a+1;
result[1] = i > a ? i+1 : a+1;
break;
}
}
return result;
}
}
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