public class Solution {
public List findDuplicates(int[] nums) {
//1 ≤ a[i] ≤ n (n = size of array),把每个元素变成负数，if判断，如果元素是负数，则出现过
List res = new ArrayList();
for(int i = 0; i < nums.length; i++) {
int index = Math.abs(nums[i]) - 1;
if(nums[index] < 0)
res.add(Math.abs(index) + 1);
nums[index] = -nums[index];
}
return res;
}
}
public class Solution { public List findDuplicates(int[] nums) {
//1 ≤ a[i] ≤ n (n = size of array),把每个元素变成负数，if判断，如果元素是负数，则出现过
List res = new ArrayList();
for(int i = 0; i < nums.length; i++) {
int index = Math.abs(nums[i]) - 1;
if(nums[index] < 0)
res.add(Math.abs(index) + 1);
nums[index] = -nums[index];
}
return res;
}
}