27. Remove Element

problem description

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.

Clarification:

Confused why the returned value is an integer but your answer is an array?

Note that the input array is passed in by reference, which means modification to the input array will be known to the caller as well.

Internally you can think of this:

// nums is passed in by reference. (i.e., without making a copy)
int len = removeElement(nums, val);

// any modification to nums in your function would be known by the caller.
// using the length returned by your function, it prints the first len elements.
for (int i = 0; i < len; i++) {
    print(nums[i]);
}

algorithm thought

题目很简单，判断相等赋值即可，只是看着自己之前写的代码实在太难受，想优化一下。

code

//之前的代码
class Solution {
public:
    int removeElement(vector<int>& nums, int val) {
        int res=nums.size();
        int pos=0;
        for(int i=0;i<nums.size();){
            if(nums[i]==val){
                i++;
                res--;
            }
            else{
                nums[pos++]=nums[i++];
            }
        }
        return res;
    }
};


//现在写出来的代码
class Solution {
public:
    int removeElement(vector<int>& nums, int val) {
        int res=0;
        for(auto num:nums){
            if(num!=val)
                nums[res++]=num;
        }
        return res;
    }
};

algorihtm analysis

上面代码是9个月前写的。但是还不知道auto情有可原，毕竟是做到后面几题才慢慢用起来的。但是循环里的处理感觉太不简洁了，看着很难受。所以改成了现在的代码，虽然时间复杂度是一样的，但是代码更加易读，看着舒服