448. find all numbers disappeared in an array

public List<Integer> (int[] nums) {
    List<Integer> res = new ArrayList<>();
    for(int i = 0; i < nums.length; i++){
        int val = Math.abs(nums[i])-1;
        if(nums[val] > 0){
            nums[val] = -nums[val];
        }else{
            res.add(val+1);
        }
    }
    return res;
}

public List<Integer> findDisappearedNumbers(int[] nums) {
    List<Integer> ret = new ArrayList<Integer>();
    for(int i = 0; i < nums.length; i++) {
        int val = Math.abs(nums[i]) - 1;
        if(nums[val] > 0) {
            nums[val] = -nums[val];
        }
    }
    for(int i = 0; i < nums.length; i++) {
        if(nums[i] > 0) {
            ret.add(i+1);
        }
    }
    return ret;
}