/*

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

*/

// Maintain two pointers, traverse form both sides.

// Since we're required to return the index of the two elements, we create an idnex mapping at first

// time: O(nlgn); space: O(n)

public class Solution {

public int[] twoSum(int[] numbers, int target) {

int[] result = new int[2];

if (numbers == null || numbers.length < 2) return result;

Map<Integer, Integer> map = new HashMap<Integer, Integer>();

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

map.put(numbers[i], i+1);

}

Arrays.sort(numbers);

int i = 0;

int j = numbers.length -1;

while (i < j){

if (numbers[i] + numbers[j] == target){

result[0] = map.get(numbers[i]);

result[1] = map.get(numbers[j]);

Arrays.sort(result);

return result;

}

else if (numbers[i] + numbers[j] < target) i++;

else j--;

}

return result;

}

}

// use map to find the index of 'target-value'

// Note that we may have two indices have the same value, so we need a list

// time: O(n); space: O(n)

public class Solution {

public int[] twoSum(int[] numbers, int target) {

if (numbers==null || numbers.length<=1)

return null;

Map<Integer, ArrayList<Integer>> map = new HashMap<Integer, ArrayList<Integer>>();

int N = numbers.length;

for (int i=0; i<N; i++){

int num = numbers[i];

if (!map.containsKey(num)){

ArrayList<Integer> lists = new ArrayList<Integer>();

map.put(num, lists);

}

map.get(num).add(i+1);

}

for (int i=0; i<N; i++){

int num = numbers[i];

if (!map.containsKey(target-num))

continue;

for (int index : map.get(target-num)){

if (index!=i+1){ // can not be itself

int[] res = new int[2];

res[0] = i+1; res[1] = index;

Arrays.sort(res);

return res;

}

}

}

return null;

}

}