1. 两数之和

给定 nums = [2, 7, 11, 15], target = 9

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

   public int[] twoSum(int[] nums, int target) {
        int indices[] = new int[2];
        for(int i = 0; i < nums.length - 1; i++){
            for(int j = i+1; j < nums.length; j++){
               if((i < j) && (nums[i] + nums[j] == target)){
                   indices[0] = i;
                   indices[1] = j;
                   break;
               }
            }
        }
        return indices;
     }

 public int[] twoSum(int[] nums, int target) {
        //浅复制，拷贝副本
        int[] copynums = new int[nums.length];
        System.arraycopy(nums, 0, copynums, 0, nums.length);
        Arrays.sort(nums);
        //
        int indices[] = new int[2];
        int start = 0;
        int end = nums.length - 1;
        int a = 0;
        int b = 0;
        while (start < end) {
            int sum = nums[start] + nums[end];
            if (sum < target) {
                start++;
            } else if (sum > target) {
                end--;
            } else if (sum == target) {
                a = nums[start];
                b = nums[end];
                break;
            }
        }
        for (int i = 0; i < copynums.length; i++) {
            if (a == copynums[i]) {
                indices[0] = i;
            } else if (b == copynums[i]) {
                indices[1] = i;
            }
        }
        return indices;
    }

  public int[] twoSum(int[] nums, int target) {
        int indices[] = new int[2];
        Map<Integer,Integer> tmpMap = new HashMap();
        for(int i = 0; i < nums.length; i++){
            if(tmpMap.containsKey(nums[i])){
                indices[0]=tmpMap.get(nums[i]);
                indices[1]=i;
                break;
            }else{
                tmpMap.put(target-nums[i],i);
            }
        }
        return indices;
    }