Algo_3_Two_Pointer

學習算法

Leetcode 167

class Solution {
public:
    vector<int> twoSum(vector<int>& numbers, int target) {
        int left = 0;
        int right = numbers.size()-1;
        int total = 0;
        while(left < right){
            total = numbers[left] + numbers[right];
            if(total == target ){
                return {left+1,right+1};
            }else if(total > target) {
                right--;
            }else{
                left++;
            }
        }
        return {-1,-1};
    }
};

Leetcode 125

#include <bits/stdc++.h>
using namespace std;
class Solution
{
public:
    bool isPalindrome(string s)
    {
        int start = 0;
        int end = s.size() - 1;
        while (start <= end)
        {
            if (!isalnum(s[start]))
            {
                start++;
                continue;
            }
            if (!isalnum(s[end]))
            {
                end--;
                continue;
            }
            if (tolower(s[start]) != tolower(s[end]))
            {
                return false;
            }else{
                start++;
                end--;
            }
        }
        return true;
    }
};

Leetcode 15

思路

使用i指針指向頭
分別使用slow和fast快慢指針去尋找和nums[i]相加為0的數
判斷假設當前數字和前一個相同，直接移動slow，避免出現重複答案

while (nums[slow] == nums[slow - 1] && slow < fast) {
  slow++;
}

#include <bits/stdc++.h>
using namespace std;
class Solution
{
public:
    vector<vector<int>> threeSum(vector<int> &nums)
    {
        vector<vector<int>> res;
        sort(nums.begin(), nums.end());
        for (int i = 0; i < nums.size(); i++)
        {
            if (i > 0 && nums[i] == nums[i - 1])
            {
                continue;
            }
            int slow = i + 1;
            int fast = nums.size() - 1;
            while (slow < fast)
            {
                int total = nums[i] + nums[slow] + nums[fast];
                if (total > 0)
                {
                    fast--;
                }
                else if (total < 0)
                {
                    slow++;
                }
                else
                {
                    res.push_back({nums[i], nums[slow], nums[fast]});
                    slow++;
                    while (nums[slow] == nums[slow - 1] && slow < fast)
                    {
                        slow++;
                    }
                }
            }
        }
        return res;
    }
};

Leetcode 11

class Solution {
public:
    int maxArea(vector<int>& height) {
        int r = height.size()-1;
        int l = 0;
        int w = 0;
        while(l < r) {
            w = max(w, (r-l)*min(height[l],height[r]));
            height[l] < height[r] ? l++ : r--;
        }
        return w;
    }
};

Leetcode 42

class Solution {
public:
    int trap(vector<int>& height) {
        int l = 0;
        int r = height.size()-1;
        int w = 0;
        int lmax = height[l];
        int rmax = height[r];
        while(l < r){
            if(lmax < rmax){
                l++;
                lmax = max(lmax,height[l]);
                w += lmax - height[l];
            }else{
                r--;
                rmax = max(rmax,height[r]);
                w += rmax - height[r];
            }
        }
        return w;
    }
};