Leetcode array 704 27 189 121 380 238 134 13

review:

704. Binary Search

class Solution {
public:
    int search(vector& nums, int target) {
        int left = 0, right = nums.size()-1;
        while(left <= right){
            int mid = left + (right-left)/2;
            if(nums[mid] == target) return mid;
            else if(nums[mid] > target){
                right = mid-1;
            }else{
                left = mid+1;
            }
        }
        return -1;

    }
};

二分搜索：

判断条件是left 和 right

这里就要注意区间开闭的问题

27. Remove Element

class Solution {
public:
    int removeElement(vector& nums, int val) {
        int slow = 0, fast = 0;
        while(fast < nums.size()){
            if(nums[fast] != val){
                nums[slow] = nums[fast];
                slow++;
            }
            fast++;
        }
        return slow;
    }
};

189. Rotate Array

class Solution {
public:
    void rotate(vector& nums, int k) {
        int n = nums.size();
        reverse(nums.begin(), nums.end());
        reverse(nums.begin(), nums.begin()+k%n);
        reverse(nums.begin()+k%n, nums.end());
    }
};

121. Best Time to Buy and Sell Stock

class Solution {
public:
    int maxProfit(vector& prices) {

        vector> dp(prices.size(), vector(2));

        //0表示未持有， 1表示持有
        dp[0][0] = 0;
        dp[0][1] = -prices[0];
        int maxVal = 0;
        for(int i=1; i

New:

380. Insert Delete GetRandom O(1)

class RandomizedSet {
private:
    vector nums;
    unordered_map valToIndex;//nums中每个元素在nums的下表
public:
    RandomizedSet() {
        
    }
    
    bool insert(int val) {
        if(valToIndex.find(val) != valToIndex.end()){
            return false;
        }
        valToIndex[val] = nums.size();
        nums.push_back(val);
        return true;
    }
    
    bool remove(int val) {
        if(valToIndex.find(val) == valToIndex.end()) return false;

        int lastNum = nums.back();
        int index = valToIndex[val];
        valToIndex[lastNum] = index;
        swap(nums[index], nums.back());
        nums.pop_back();
        valToIndex.erase(val);
        return true;
    }
    
    int getRandom() {
        if(nums.size() == 0) return false;
        return nums[rand() % nums.size()];
    }
};

因为需要运行时间时O（1），所以要加上map

删除的时候要把需要删除的变到最后，再直接pop掉

rand用法：

1.rand()

2. nums[rand() % nums.size()];

3. left+rand() % (right-left)

238. Product of Array Except Self

class Solution {
public:
    vector productExceptSelf(vector& nums) {
        int n = nums.size();
        vector presuf(n, nums[0]);
        for(int i=1; i postsuf(n, nums[n-1]);
        for(int i=n-2; i>=0; i--){
            postsuf[i] = postsuf[i+1] * nums[i];
        }

        vector res(n);
        res[0] = postsuf[1];
        res[n-1] = presuf[n-2];
        for(int i=1; i

134. Gas Station

class Solution {
public:
    int canCompleteCircuit(vector& gas, vector& cost) {
        int sum = 0, minSum = 0;
        int start = 0;
        for(int i=0; i

class Solution {
public:
    int canCompleteCircuit(vector& gas, vector& cost) {
        int sum = 0;
        int start = 0;
        for(int i=0; i

13. Roman to Integer

class Solution {
public:
    int romanToInt(string s) {
        unordered_map map;
        map['I'] = 1;
        map['V'] = 5;
        map['X'] = 10;
        map['L'] = 50;
        map['C'] = 100;
        map['D'] = 500;
        map['M'] = 1000; 
        int sum = 0;
        for(int i = 0; i