# 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

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]

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:

posted @ 2018-04-24 09:47  Zhao_Gang  阅读(94)  评论(0编辑  收藏  举报