75. Sort Colors(荷兰国旗问题 三指针)

 

Given an array with n objects colored red, white or blue, sort them so that objects of the same color are adjacent, with the colors in the order red, white and blue.

Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.

Note:
You are not suppose to use the library's sort function for this problem.

 

 

三个指针 lo =cur =0,hi =n-1

a[cur]==0: if(lo==cur) cur++ lo++

　　　　　else: swap(lo,cur) lo++

 

 

class Solution {
public:
    void swap(vector<int>& nums,int i ,int j) {
        int a = nums[i];
        nums[i] = nums[j];
        nums[j] = a;
    }
    void sortColors(vector<int>& nums) {
        int ptr = 0;
        for(int i = 0;i < nums.size();i++) {
            if(nums[i]==0) {
                swap(nums,i,ptr);
                ptr++;
            }
        }
        for(int i = ptr;i < nums.size();i++){
            if(nums[i]==1) {
                swap(nums,i,ptr);
                ptr++;
            }
        }
    }
};

 

 

class Solution {
public:
    void swap(vector<int>& nums,int i ,int j) {
        int a = nums[i];
        nums[i] = nums[j];
        nums[j] = a;
    }
    void sortColors(vector<int>& nums) {
        int low = 0;
        int high = nums.size()-1;
        int i = 0;
        while(i<=high) {
            if(nums[i]==0) {
                swap(nums,low,i);
                i++;
                low++;
            } else if (nums[i]==2){
                swap(nums,high,i);
                high--;

            } else {
                i++;
            }
        }
    }
};

 

 

 

class Solution {
    public void sortColors(int[] nums) {
        int begin  = 0;
        int cur = 0;
        int end = nums.length-1;
        while(cur<=end){
            if(nums[cur]==2){
                swap(nums,cur,end);
                end--;
            }
            else if(nums[cur]==1){
                cur++;
            }
            else //(nums[cur]==0)
            {
                if(nums[cur]!=nums[begin])
                    swap(nums,cur,begin);
                begin++;
                cur++;
            }
        }
    }
    private void swap(int[] nums,int i ,int j){
        int temp;
        temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
}