第81天。

今天的题目是Remove Element:

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 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:

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

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

简单的我们可以遍历数组，然后找到和val相同的元素，然后删除，但是这样对于多个相同的元素效率不高，所以我们先不删除元素，而是把他移动到最后面去，知道遍历完才删除。

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int removeElement(vector<int>& nums, int val) {
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    int last = nums.size();
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    for(int i = 0;i < last;i++) {
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        if (nums[i] == val) swap(nums[--last],nums[i--]);
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    }
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    nums.erase(nums.begin() + last,nums.end());
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    return nums.size();
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}

dicuss中另一个方法也很精妙：

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int removeElement(int A[], int n, int elem) {
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    int begin=0;
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    for(int i=0;i<n;i++) if(A[i]!=elem) A[begin++]=A[i];
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    return begin;
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}