LeetCode 167. Two Sum II - Input array is sorted

problem:

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

 

 

first try:

class Solution {
    public int[] twoSum(int[] numbers, int target) {
        //find the first number samller than the target
        int index2 =0;
        for(int i=numbers.length-1; 0<=i;i--){
            if(numbers[i]<target){
                index2 = i+1;
                break;
            }
        }
        
        int index1 =0;
        for(int i=0; i<index2; i++) {
            if(numbers[i]+numbers[index2] == target){
                index1 = i+1;
                break;
            }
        }
        
        int[] output = new int[2];
        output[0] = index1;
        output[1] = index2;
        return output;
    }
}

result:

 

Submission Result: Runtime Error 
Runtime Error Message: Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3
    at Solution.twoSum(Solution.java:14)
    at __Driver__.main(__Driver__.java:23)
Last executed input: [2,3,4]
6

又是数组越界。

 

second try：

class Solution {
    public int[] twoSum(int[] numbers, int target) {
        //find the first number samller than the target
        int index2 =0;
        for(int i=numbers.length-1; 0<=i;i--){
            if(numbers[i]<target){
                index2 = i+1;
                break;
            }
        }
        
        int index1 =0;
        for(int i=0; i<index2; i++) {
            if(numbers[i]+numbers[index2-1] == target){
                index1 = i+1;
                break;
            }
        }
        
        int[] output = new int[2];
        output[0] = index1;
        output[1] = index2;
        return output;
    }
}

 

result:

Submission Result: Wrong Answer 
Input: [-1,0]
-1
Output: [0,0]
Expected: [1,2]

 

应考虑负数和0的情况。

third try：

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

        //two pointers
        int index1 =0, index2 =numbers.length-1;
        
        while(index1<index2) {
            if(numbers[index1]+numbers[index2]==target){
                break;
            }else if(numbers[index1]+numbers[index2]<target){
                index1++;
            }else if(target<(numbers[index1]+numbers[index2])){
                index2--;
            }
        }
        
        int[] output = new int[2];
        output[0] = index1+1;
        output[1] = index2+1;
        return output;
    }
}

 

result:

 

anylysis:

典型的双指针问题。