26-remove_duplicates_from_sorted_array.js

/* Given an integer array nums sorted in non-decreasing order, 
 * remove the duplicates in-place such that each unique element appears only once. 
 * The relative order of the elements should be kept the same.
 * 
 * Since it is impossible to change the length of the array in some languages, 
 * you must instead have the result be placed in the first part of the array nums. 
 * More formally, if there are k elements after removing the duplicates, 
 * then the first k elements of nums should hold the final result. 
 * It does not matter what you leave beyond the first k elements.
 * 
 * Return k after placing the final result in the first k slots of nums.
 * 
 * Do not allocate extra space for another array. 
 * You must do this by modifying the input array in-place with O(1) extra memory.
 * 
 * Example 1: 
 * Input: nums = [1,1,2]
 * Output: 2, nums = [1,2,_]
 * Explanation: Your function should return k = 2, with the first two elements of nums being 1 and 2 respectively.
 * It does not matter what you leave beyond the returned k (hence they are underscores).
 * 
 * Example 2:
 * Input: nums = [0,0,1,1,1,2,2,3,3,4]
 * Output: 5, nums = [0,1,2,3,4,_,_,_,_,_]
 * Explanation: Your function should return k = 5, with the first five elements of nums being 0, 1, 2, 3, and 4 respectively.
 * It does not matter what you leave beyond the returned k (hence they are underscores).
 * 
 * Constraints:
 * 
 * 1 <= nums.length <= 3 * 10^4
 * -100 <= nums[i] <= 100
 * nums is sorted in non-decreasing order.
 */

var removeDuplicates = function (nums) {
    if (nums.length === 0) return 0;
    let first = 0;
    let second = 1;
    while (second < nums.length) {
        if (nums[second] !== nums[first]) {
            first++;
            nums[first] = nums[second];
            second++;
        } else {
            second++;
        }
    }
    return first + 1;
};

/* Explanation:
 * If nums is empty, return 0.
 * Create two pointers, first equal to 0 and second equal to 1.
 * Loop through nums while second is less than nums.length.
 * If second is not equal to first,
 * increament first,
 * set first equal to second
 * and then increament second.
 * Otherwise, just increament second.
 * After second has gone through all of nums, return the number of unique numbers, which is first + 1.
 */