LeetCode 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:

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Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

Solution 1: HashMap O(n); O(n)

Build a HashMap between number and its index.
For each number n in the array, check if target - n exists in map.
If exists, return the indices, otherwise, put n and its index into map.
Return {-1, -1} if not found.

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

Contents
  1. 1. Solution 1: HashMap O(n); O(n)
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