1 twoSum

package LeetCode;import java.util.Arrays;import java.util.HashMap;/*要求：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. *给一个数组然后还有一个目标值，输出相加等于目标值的两个数组的下标*/public class TwoSum {    /**     * 这个方案最简单，刚开始被系统判断为超时不通过,改进之后就通过     *      * @param nums     * @param target     * @return     */    public static int[] twoSum1(int[] nums, int target) {        if (nums == null)            return null;        int[] result = new int[2];        for (int i = 0; i < nums.length - 1; i++) {            for (int j = i + 1; j < nums.length; j++) {                if (nums[i] + nums[j] == target) {                    result[0] = i;                    result[1] = j;                }            }        }        return result;    }    /**     * 这种方法只是第一种方法变成了减法，但是通过，不知什么原因。     *      * @param nums     * @param target     * @return     */    public static int[] twoSum2(int[] nums, int target) {        int[] answer = new int[2];        for (int i = 0; i < nums.length; ++i) {            answer[0] = i;            int b = target - nums[i];            for (int j = i + 1; j < nums.length; ++j) {                if (nums[j] == b) {                    answer[1] = j;                    return answer;                }            }        }        return null;    }    /**     * 这是hashMap的做法，时间复杂度变为o（n）     *      * @param nums     * @param target     * @return     */    public static int[] twoSum3(int[] nums, int target) {        int[] answer = new int[2];        HashMap<Integer, Integer> map = new HashMap<>();        for (int i = 0; i < nums.length; ++i) {            map.put(nums[i], i);        }        for (int i = 0; i < nums.length; ++i) {            int b = target - nums[i];            if (map.containsKey(b) && i != map.get(b))                return new int[] { i, map.get(b) };        }        return answer;    }    /**     *      * @param nums     * @param target     * @return     */    public static int[] twoSum4(int[] nums, int target) {        int numsLength = nums.length;        if (numsLength < 2) {            return nums;        }        int[] result = new int[2];        HashMap<Integer, Integer> numsInDictionary = new HashMap<>();        for (int i = 0; i < numsLength; i++) {            if (numsInDictionary.containsKey(target - nums[i])) {                result[1] = i;                result[0] = numsInDictionary.get(target - nums[i]);                break;            }            if (!numsInDictionary.containsKey(nums[i])) {                numsInDictionary.put(nums[i], i);            }        }        return result;    }    /**     *      * @param nums     * @param target     * @return     */    public static int[] twoSum5(int[] nums, int target) {        if (nums == null)            return null;        int[] nums2 = Arrays.copyOf(nums, nums.length);        Arrays.sort(nums2);        int a = 0, b = 0;        int start = 0, end = nums2.length - 1;        // find two nums        while (start < end) {            int sum = nums2[start] + nums2[end];            if (sum < target)                start++;            else if (sum > target)                end--;            else {                a = nums2[start];                b = nums2[end];                break;            }        }        // find the index of two numbers        int[] res = new int[2];        for (int i = 0; i < nums.length; i++) {            if (nums[i] == a) {                res[0] = i;                break;            }        }        if (a != b) {            for (int i = 0; i < nums.length; i++) {                if (nums[i] == b) {                    res[1] = i;                    break;                }            }        } else {            for (int i = 0; i < nums.length; i++) {                if (nums[i] == b && i != res[0]) {                    res[1] = i;                    break;                }            }        }        return res;    }    public static void main(String[] args) {        int[] n = new int[] { 3, 2, 4 };        System.out.println(Arrays.toString(twoSum5(n, 6)));    }}