167. Two Sum II - Input array is sorted

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.

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

使用第一题的方法依然是可行的,时间/空间复杂度是O(n)。

var twoSum = function(numbers, target) {
    var map = {};
    var num = numbers.length;
    for (var i = 0; i<num;i++) {
        if (map[target-numbers[i]]!==undefined)
            return [map[target-numbers[i]]+1,i+1];
        map[numbers[i]] = i;
    }
};

不过如果好好利用已经被排为升序这个信息,我们可以找到更加优化的算法。
因为数组已被排序,且一定存在解。那么我们可以从两边往中间找。

var twoSum = function(numbers, target) {
    var left = 0;
    var right = numbers.length-1;
    while(true) {
        if ((numbers[left]+numbers[right])===target) 
            return [left+1,right+1];
        else if ((numbers[left]+numbers[right])>target)
            right--;
        else
            left++;
    }
};

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