LeetCode-Easy部分中标签为 Binary Search 167. Two Sum II - Input array is sorted

原题

Given an array of integers that is already sorted in ascending order, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution and you may not use the same element twice.

Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

题目分析

在升序数组中，求两个数的目标值所在的索引。

代码实现

1. 哈希表

        public int[] TwoSum(int[] numbers, int target)
        {
            if (numbers.Length == 0) 
                return new int[] { };

            int index = searchInsertIndex(numbers, target);
            //值和索引
            Dictionary<int, int> dict = new Dictionary<int, int>();
            for(int i=0;iif (dict.ContainsKey(target - numbers[i]))
                    return new int[2] { dict[target - numbers[i]]+1, i+1 };
                else
                {
                    if (!dict.ContainsKey(numbers[i]))
                        dict.Add(numbers[i], i);
                }

            }
            return new int[] { };
        }

1. two pointers
   根据有序数组的规律，不断调整起、终指针。

        public int[] TwoSum2(int[] num, int target)
        {
            //因为一定存在解，所以不做边界检查
            int left = 0, right = num.Length - 1;
            while (left < right)
            {
                int v = num[left] + num[right];
                if (v == target)
                    return new int[2] { left + 1, right + 1 };
                else if (v > target)
                    right--;
                else
                    left++;
            }
            return new int[] { };
        }
}