Two Sum

暴力法：

public class Solution {
    public static int[] twoSum(int[] nums, int target) {
        int[] result = new int[2];
        for (int i = 0; i < nums.length; i++) {
            for (int j = 0; j < nums.length; j++) {
                if (i!=j&&nums[i] + nums[j] == target) {
                    result[0] = i;
                    result[1] = j;
                    return result;
                }
            }
        }

        return null;
    }

    public static void main(String[] args) {
        int[] nums = new int[]{2, 7, 11, 15};
        int target = 9;
        int[] result = twoSum(nums, target);
        System.out.println("[" + result[0] + "," + result[1] + "]");
    }
}

先排序，后一次遍历，时间复杂度O(NlogN)

import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

public class Solution {
    //总时间复杂度O(nlogn)
    public static int[] twoSum(int[] nums, int target) {
        int[] result = new int[2];
        int len = nums.length;

        //(1)保存原有数组的下标索引，时间复杂度O(n)
        Map<Integer, Integer> indexMap = new HashMap<Integer, Integer>();
        //数组中有重复的情况，如：nums=[3,3] target=6
        Map<Integer, Integer> duplicateMap = new HashMap<Integer, Integer>();
        for (int i = 0; i < len; i++) {
            if (indexMap.containsKey(nums[i])) {
                duplicateMap.put(nums[i], i);
            } else {
                indexMap.put(nums[i], i);
            }
        }

        //(2) 排序 O(nlogn)
        Arrays.sort(nums);

        //（3）一次遍历O(n)
        int tmp;
        for (int i = 0, j = len - 1; i < j; ) {
            tmp = nums[i] + nums[j];
            if (tmp == target) {
                if (nums[i] == nums[j]) {
                    result[0] = indexMap.get(nums[i]);
                    result[1] = duplicateMap.get(nums[j]);

                    if (result[0] > result[1]) {
                        tmp = result[0];
                        result[0] = result[1];
                        result[1] = tmp;
                    }
                } else {
                    result[0] = indexMap.get(nums[i]);
                    result[1] = indexMap.get(nums[j]);

                    if (result[0] > result[1]) {
                        tmp = result[0];
                        result[0] = result[1];
                        result[1] = tmp;
                    }
                }
                return result;
            } else if (tmp > target) {
                j--;
            } else {
                i++;
            }

        }
        return null;
    }

    public static void main(String[] args) {
        int[] nums = new int[]{3, 3};
        int target = 6;
        int[] result = twoSum(nums, target);
        System.out.println("[" + result[0] + "," + result[1] + "]");
    }
}

两次遍历：

import java.util.HashMap;
import java.util.Map;

public class Solution {
    //总时间复杂度O(n),空间复杂度O(n)
    public static int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int i = 0; i < nums.length; i++) {
            map.put(nums[i], i);
        }
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];
            if (map.containsKey(complement) && map.get(complement) != i) {
                return new int[]{i, map.get(complement)};
            }
        }
        return null;
    }

    public static void main(String[] args) {
        int[] nums = new int[]{3, 3};
        int target = 6;
        int[] result = twoSum(nums, target);
        System.out.println("[" + result[0] + "," + result[1] + "]");
    }
}

一次遍历：

import java.util.HashMap;
import java.util.Map;

public class Solution {
    //总时间复杂度O(n),空间复杂度O(n)
    public static int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];
            if (map.containsKey(complement)) {
                return new int[]{map.get(complement), i};
            }
            map.put(nums[i], i);
        }
        return null;
    }

    public static void main(String[] args) {
        int[] nums = new int[]{3, 3};
        int target = 6;
        int[] result = twoSum(nums, target);
        System.out.println("[" + result[0] + "," + result[1] + "]");
    }
}