80. Remove Duplicates from Sorted Array II

Follow up for "Remove Duplicates":What if duplicates are allowed at most twice?
For example,Given sorted array nums = [1,1,1,2,2,3],Your function should return length = 5, with the first five elements of nums being 1, 1, 2, 2 and 3. It doesn't matter what you leave beyond the new length.

这里有两个方法，都是从头到尾走一遍数组，第一种办法用了一个标志位来判断该元素是否重复两次
后面的方法很巧妙，直接用当前要判断的数与已经判断过的倒数第二个数来比较，现在有这么两种情况，最后两个数可能是2,2或2,3。现在的数可能是2或3或4，那么只要保证当前数大于倒数第二个数，当前数就可以放进已判断数组。

var removeDuplicates = function(nums) {
    var num = nums.length;
    if (num < 3)
        return num;
    var flag = false;
    var tail = 0;
    for (var i = 0; i < num;i++) {
        if (nums[i]!==nums[i+1]&&flag) {
            nums[tail++] = nums[i-1];
            nums[tail++] = nums[i];
            flag = false;
        }else if (nums[i]!==nums[i+1]&&!flag) {
            nums[tail++] = nums[i];
            flag = false;
        } else if (nums[i]===nums[i+1]&&!flag) {
            flag = true;
        }
    }
    return tail;
};

var removeDuplicates = function(nums) {
    var num = nums.length;
    if (num < 3)
        return num;
    var k = 2;
    for(var i = 2;inums[k-2])
            nums[k++] = nums[i];
    }
    return k;
};