Solution2.java

// 167. Two Sum II - Input array is sorted
// https://leetcode.com/problems/two-sum-ii-input-array-is-sorted/description/
//
// 二分搜索法
// 时间复杂度: O(nlogn)
// 空间复杂度: O(1)
public class Solution2 {

    public int[] twoSum(int[] numbers, int target) {

        if(numbers.length < 2 /*|| !isSorted(numbers)*/)
            throw new IllegalArgumentException("Illegal argument numbers");

        for(int i = 0 ; i < numbers.length - 1 ; i ++){
            int j = binarySearch(numbers, i+1, numbers.length-1, target - numbers[i]);
            if(j != -1){
                int[] res = {i+1, j+1};
                return res;
            }
        }

        throw new IllegalStateException("The input has no solution");
    }

    private int binarySearch(int[] nums, int l, int r, int target){

        if(l < 0 || l > nums.length)
            throw new IllegalArgumentException("l is out of bound");

        if(r < 0 || r > nums.length)
            throw new IllegalArgumentException("r is out of bound");

        while(l <= r){
            int mid = l + (r - l)/2;
            if(nums[mid] == target)
                return mid;
            if(target > nums[mid])
                l = mid + 1;
            else
                r = mid - 1;
        }

        return -1;
    }

    private boolean isSorted(int[] numbers){
        for(int i = 1 ; i < numbers.length ; i ++)
            if(numbers[i] < numbers[i-1])
                return false;
        return true;
    }

    private static void printArr(int[] nums){
        for(int num: nums)
            System.out.print(num + " ");
        System.out.println();
    }

    public static void main(String[] args) {

        int[] nums = {2, 7, 11, 15};
        int target = 9;
        printArr((new Solution2()).twoSum(nums, target));
    }
}