LeetCode-两数之和

给定一个整数数组和一个目标值，找出数组中和为目标值的两个数。

你可以假设每个输入只对应一种答案，且同样的元素不能被重复利用。

示例:

给定 nums = [2, 7, 11, 15], target = 9

因为 nums[0] + nums[1] = 2 + 7 = 9
所以返回 [0, 1]

思路：

（1）暴力解法。对于任意num in nums[],遍历数组查找能否构成target；复杂度为O(n2)

（2）两头齐头并进。首先先进行排序，然后 若最左+最右>target，左边指针向右移动；反之向左移动；复杂度O(nlogn)

（3）map映射法。首先将nums[]存入map中，然后遍历nums,查找target的差值

import java.io.File;
import java.io.FileNotFoundException;
import java.lang.reflect.Array;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import java.util.stream.IntStream;

import javax.lang.model.type.PrimitiveType;
import javax.print.attribute.standard.PrinterLocation;

import org.omg.CORBA.ORBPackage.InconsistentTypeCode;

public class Main {
	public Main() {
		// TODO Auto-generated constructor stub
	}

	public static void main(String[] args) throws FileNotFoundException {
		Solution solution = new Solution();
		int nums[] = { 7, 2, 9, 11 };
		int result[] = solution.twoSumMap(nums, 13);

		for (int i = 0; i < result.length; ++i) {
			System.out.println(result[i]);
		}

	}

}

class Solution {

	class ArrayHelper {
		private int index;
		private int value;

		public ArrayHelper(int index, int value) {
			this.index = index;
			this.value = value;
		}

	}

	public int[] twoSum(int[] nums, int target) {
		int num1, num2;
		for (int i = 0; i < nums.length; ++i) {
			num1 = nums[i];
			for (int j = i + 1; j < nums.length; ++j) {
				num2 = nums[j];
				if (num1 + num2 == target) {
					return new int[] { i, j };
				}
			}

		}

		return new int[] {};

	}

	public int[] twoSumNLogN(int[] nums, int target) {
		int length = nums.length;
		ArrayHelper[] helpers = new ArrayHelper[length];
		for (int i = 0; i < length; ++i) {
			helpers[i] = new ArrayHelper(i, nums[i]);
		}
		Arrays.sort(helpers, new Comparator<ArrayHelper>() {

			@Override
			public int compare(ArrayHelper o1, ArrayHelper o2) {
				if (o1.value > o2.value) {

					return 1;
				}
				// TODO Auto-generated method stub
				else {
					return -1;

				}
			}
		});

		for (int left = 0, right = length - 1; left < right;) {
			while (helpers[left].value + helpers[right].value > target) {
				--right;
			}
			while (helpers[left].value + helpers[right].value < target) {
				++left;
			}

			if (helpers[left].value + helpers[right].value == target) {
				return new int[] { helpers[left].index, helpers[right].index };
			}
		}
		return new int[] {};

	}

	public int[] twoSumMap(int[] nums, int target) {
		int length = nums.length;
		int subtraction;

		Map<Integer, Integer> map = new HashMap<>();
		for (int i = 0; i < length; ++i) {
			map.put(nums[i], i);

		}

		for (int j = 0; j < length; ++j) {
			subtraction = target - nums[j];
			
			if (map.containsKey(subtraction) && map.get(subtraction) != j) 
			{

				return new int[]{j,map.get(subtraction)};
			}
		}

		return new int[] {};

	}
}