[LeetCode]1 Two Sum

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

// Solution A
// return twoSum_SortAndTwoPointer(numbers, target);

// Solution B
return twoSum_Map(numbers, target);
}

/////////////////////////////
// Solution 2: Map
//
// Map[target - value, index]
public int[] twoSum_Map(int[] numbers, int target) {
Map<Integer, Integer> map = new HashMap<>();
for (int i = 0 ; i < numbers.length ; i ++)
{
Integer j = map.get(numbers[i]);
if (j != null)
{
i++;
j++;
return new int[] {Math.min(i, j), Math.max(i, j)};
}
else
{
map.put(target - numbers[i], i);
}
}
return null;
}

/////////////////////////////
// Solution 1: Sort and iteration with 2 index
//
// 数组排序
// 使用两个index分别从头和尾遍历
// 此方法可扩展至three sum， four sum

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

int len = numbers.length;
List<IndexValue> nums = new ArrayList<>(len);
for (int i = 0 ; i < len ; i ++)
{
nums.add(new IndexValue(i, numbers[i]));
}

// Sort
Collections.sort(nums, new Comparator<IndexValue>()
{
public int compare(IndexValue a, IndexValue b)
{
return Integer.compare(a.value, b.value);
}
});

int start = 0;
int end = len - 1;
while (start < end)
{
int sum = nums.get(start).value + nums.get(end).value;
if (sum == target)
{
int a = nums.get(start).index + 1;
int b = nums.get(end).index + 1;
return new int[] {Math.min(a, b), Math.max(a, b)};
}
else if (sum < target)
{
start++;
}
else
{
end--;
}
}
return null;
}

private static class IndexValue
{
int index;
int value;
IndexValue(int index, int value)
{
this.index = index;
this.value = value;
}
}
}