【LeetCode从零单排（Java）】No1.Two Sum

题目

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].

暴力枚举法

public class Solution {
public int[] twoSum(int[] nums, int target) {

for(int i = 0;i<nums.length ;i++ )
{
for(int m = 0;m<nums.length ;m++ )
{

if(nums[i] +nums[m] ==  target &&m!= i)
{
int[] arr = new int[2];
arr[0]= i;
arr[1]= m;
return arr;
}

}

}

return null;

}
}

//使用hash表

public class Solution {
public int[] twoSum(int[] nums, int target) {

int[] result = new int[2];
int n;
Integer idx;
Map<Integer, Integer> map = new HashMap<Integer, Integer>(nums.length);

for (int i = 0; i < nums.length; i++) {
map.put(nums[i], i);
}
for (int i = 0; i < nums.length - 1; i++) {
n = target - nums[i];
idx = map.get(n);
if(idx != null && idx > i){
result[0] = i;
result[1] = idx;
break;
}
}
return result;
}
}

使用第二种方法可大幅提高算法时间