快慢指针在数组中的应用

题目

1、remove-duplicates-from-sorted-array

Given a sorted array, remove the duplicates in place such that each element appear only once and return the new length.
Do not allocate extra space for another array, you must do this in place with constant memory.
For example,
Given input array A =[1,1,2],
Your function should return length =2, and A is now[1,2].

2、remove-duplicates-from-sorted-array-ii

Follow up for “Remove Duplicates”:
What if duplicates are allowed at most twice?
For example,
Given sorted array A =[1,1,1,2,2,3],
Your function should return length =5, and A is now[1,1,2,2,3].

代码

题目1代码

class Solution {
public:
    int removeDuplicates(int A[], int n) {
        if(n<=1) return n;
        int count=1;
        for(int i=1;i<n;i++){
            if (A[i]!=A[count-1])
                A[count++]=A[i];
        }
        return count;
    }
};

题目2代码

class Solution {
public:
    int removeDuplicates(int A[], int n) {
        if(n<=2) return n;
        int count=2;
        for(int i=2;i<n;i++){
            if(A[i]!= A[count-2])
                A[count++]=A[i];
        }
        return count;
    }
};

思路

使用快慢两个“指针”，慢指针count指向新数组末尾的下一个位置，快指针i遍历原数组，当快指针检测到某个数符合条件时，把这个数赋值给慢指针，然后快慢指针均向右移动一格，直到快指针遍历完数组，返回慢指针的值即为长度。