I have written an algorithm for "Remove Duplicates from Sorted Array" challenge on leetcode. I ran the code on google dev tools and nothing was wrong. But leetcode says it is. Can somebody point out why is it so? Is it because I didn't fulfil the condition "you must do this by modifying the input array in-place with O(1) extra memory."? I haven't learnt big O notation yet, if so.

var removeDuplicates = function(nums) {
    for (let a of nums){
        if (nums[nums.indexOf(a)] ==  nums[nums.lastIndexOf(a)] && nums.indexOf(a) !== nums.lastIndexOf(a)) {
        nums.splice(nums.indexOf(a), nums.lastIndexOf(a)-nums.indexOf(a)) }
    }
    return nums;
};