Two Sum

题目：

Given an array of integers, 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.

the input array is already sorted in ascending order

解答：

第一种解法是二分查找

public int[] twoSum(int[] numbers, int target) {
    // Assume input is already sorted.
    for (int i = 0; i < numbers.length; i++) {
        int j = bsearch(numbers, target - numbers[i], i + 1);
        if (j != -1) {
            return new int[] { i + 1, j + 1 };
        }
    }
    throw new IllegalArgumentException("No two sum solution");
}

private int bsearch(int[] A, int key, int start) {
    int L = start, R = A.length - 1;
    while (L < R) {
        int M = (L + R) / 2;
        if (A[M] < key) {
            L = M + 1;
        } else {
            R = M;
        }
    }
    
    return (L == R && A[L] == key) ? L : -1;
}

第二种解法使用两个游标

public int[] twoSum(int[] numbers, int target) {
    // Assume input is already sorted.
    int i = 0, j = numbers.length - 1;

    while (i < j) {
        int sum = numbers[i] + numbers[j];
        if (sum < target) {
            i++;
        } else if (sum > target) {
            j--;
        } else {
            return new int[] { i + 1, j + 1 };
        }
    }
 
    throw new IllegalArgumentException("No two sum solution");
}