27. 移除元素

一、题意
Given an array nums and a value val, 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 by modifying the input array in-place with O(1) extra memory.

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

Example 1:

Given nums = [3,2,2,3], val = 3,

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

It doesn’t matter what you leave beyond the returned length.
Example 2:

Given nums = [0,1,2,2,3,0,4,2], val = 2,

Your function should return length = 5, with the first five elements of nums containing 0, 1, 3, 0, and 4.

Note that the order of those five elements can be arbitrary.

It doesn’t matter what values are set beyond the returned length.

二、分析和解答

 public int removeElement(int[] nums, int val) {
        if(nums.length == 0)
            return 0;
        int p = 0,q = 0;
        int num = 0;
        while(p<nums.length && q<nums.length){
            while(p < nums.length && nums[p] != val){         
                p++;
            }
            q = p;
            while(q < nums.length && nums[q] == val){
                q++;
            }

            if(p < nums.length && q < nums.length){
                nums[p] = nums[p] ^ nums[q];
                nums[q] = nums[p] ^ nums[q];
                nums[p] = nums[p] ^ nums[q];    
            }
            p++;
        }

        for(int i=nums.length-1;i>=0 && nums[i]==val;i--){
            num++;
        }
        return nums.length - num;
    }

注意：
（1）while(p < nums.length && nums[p] != val){ p++; }
中，一定要有p < nums.length这个判断。因为里面是p++，最后一次导致p的数为nums.length，而且此时也能进入while的判断，数组越界！
（2）原地变换，使用的是异或操作！
（3）使用快慢指针，慢指针指向等于val的值，快指针等于这个值后面的不等于val的值！

别人的方法：

    public int removeElement(int[] nums, int val) {
        if(nums.length == 0)
            return 0;
        int count = 0;
        for(int i=0;i<nums.length;i++){
            if(nums[i] != val)
                nums[count++] = nums[i];
        }
        return count;
    }

虽然别人的看着简单，貌似我的要比它的快点！也不一定，可能那时候服务器压力小。