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].

注意不同的index上相同的数字，本想用map存index，但是同样的数字存进去会覆盖以前的数字

time o(nlogn) space . o(n)

class Solution {
    public int[] twoSum(int[] nums, int target) {
        if(nums == null || nums.length == 0)
            return new int[0];
        
        int[] nums2 = Arrays.copyOf(nums,nums.length);
        
        Arrays.sort(nums2);
        
        int[] res = new int[2];
        
        int i = 0, j = nums2.length-1;
        int a = 0, b = 0;
        
        while(i < j){
            int sum = nums2[i] + nums2[j];
            
            if(sum < target){
                i++;
            }
            else if(sum > target){
                j--;
            }
            
            else{
                a = nums2[i];
                b = nums2[j];
                break;
            }
        }
        
        if(a != b){
            for(int k = 0; k < nums.length; k++){
                if(nums[k] == a){
                    res[0] = k;
                }
                
                if(nums[k] == b){
                    res[1] = k;
                }
            }
        }
        
        if(a == b){
            int m = 0;
            for(m = 0; m < nums.length; m++){
                if(nums[m] == a){
                    res[0] = m;
                    break;
                }
            }
            
            for(int n = m+1; n < nums.length; n++){
                if(nums[n] == b){
                    res[1] = n;
                    break;
                }
            }
        }
        
        Arrays.sort(res);
        
        return res;
    }
}

time o(n) space o(n)

class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int[] result = new int[2];
    Map<Integer, Integer> map = new HashMap<Integer, Integer>();
    for (int i = 0; i < numbers.length; i++) {
        if (map.containsKey(target - numbers[i])) {
            result[1] = i;
            result[0] = map.get(target - numbers[i]);
            return result;
        }
        map.put(numbers[i], i );
    }
    return result;
    }
}