27. Remove Element

一、问题：

Given an array and a value, remove all instances of that value in place and return the new length.

Do not allocate extra space for another array, you must do this in place with constant memory.

The order of elements can be changed. It doesn't matter what you leave beyond the new length.

Example:
Given input array nums = [3,2,2,3], val = 3

Your function should return length = 2, with the first two elements of nums being 2.

二、解题一：

class Solution {
public:
    int removeElement(vector<int>& nums, int val)
    {
        /*
        思路是 把指定元素全部移到数组后面去，
        如果恰好后面元素是val,n仍然减小。
        */

        int i = 0;
        int n = nums.size();
        while (i < n) {
            if (nums[i] == val) {
                nums[i] = nums[n - 1];
                n--;
            }
            else
                i++;
        }
        return n;
    }

};

解题二：

class Solution {
    /*
    思路是将不从重复的元素，依次移动到最前面，
    begin指向最后一个不重复元素的位置。
    */
public:
    int removeElement(vector<int>& nums, int val)
    {
        int n = nums.size();
        int begin = 0;
        for (int i = 0; i<n; i++) if (nums[i] != val) nums[begin++] = nums[i];
        return begin;
    }

};