# Index or value based 2-pointer method?

• I sense this question is somehow related to slow/fast pointer methods. However, as I was trying to approach this question with a method similar to Linked List Cycle question, I was trying to do p1=p1+1, and p2 = p2 +2 approach, as blow. Because in the Linked List Cycle problem, we each time move the slow & fast nodes based on the index (or more precisely, links, for linked list). Obviously it did not worked here. (past most test case though). Compared with @echoxiaolee solution. I am wondering why are we moving the slow & fast pointers based on values nums[p] = nums[nums[p]], not based on index like what I did?

``````public class Solution {
public int findDuplicate(int[] nums) {
int p1= 0;
int p2 = 0;
do{
if(p1+1 == nums.length){
p1 = 0;
}else{
p1++;
}
if(p2+2 == nums.length){
p2 = 0;
}else if(p2 + 2 == nums.length + 1){
p2 = 1;
}else{
p2++;
p2++;
}
}while(nums[p1] != nums[p2] || p2==p1 ) ;
return nums[p1];
}
}
``````

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