LeetCode-Two Sum

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

翻译过来就是求两数之和等于给定的数字,注意数组是无序的

方案一：两重循环，时间复杂度O(n^2) public int[] twoSum(int[] nums, int target) {
for (int i = 0; i < nums.length; i++) {
for (int j = i + 1; j < nums.length; j++) {
if (target == nums[i] + nums[j]) {
return new int []{ i + 1, j + 1};
}
}
}
return null;
}方案二：使用HashMap,时间复杂度O(n)

public int[] twoSum(int[] nums, int target) {
Map<Integer,Integer> map = new HashMap<>();
for (int i = 0; i < nums.length; i++) {
if (map.containsKey(nums[i])) {
int v = map.get(nums[i]);
map.put(nums[i],v < i ? v : i);
} else {
map.put(nums[i],i);
}
}
for (int i = 0; i < nums.length; i++) {
int t = target - nums[i];
if (map.containsKey(t)) {
int v = map.get(t);
if (v != i) {
if (v < i) {
return new int [] {v + 1, i + 1};
} else {
return new int [] {i + 1, v + 1};
}
}
}
}
return null;
}