LeetCode热题100刷题16：74. 搜索二维矩阵、33. 搜索旋转排序数组、153. 寻找旋转排序数组中的最小值、98. 验证二叉搜索树

74. 搜索二维矩阵

cpp 复制代码

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        int row = matrix.size();
        int col = matrix[0].size();

        for(int i=0;i<row;i++) {
            //先排除一下不存在的情况
            if(i>0&&matrix[i][0]>target && matrix[i-1][col-1]<target)
                return false;
            //锁定某一行之后使用二分查找
            if(matrix[i][0] <=target && matrix[i][col-1]>=target) {
                int begin=0,end=col-1;
                while(begin<=end) {
                    int mid = begin + (end-begin)/2;
                    if(matrix[i][mid] > target) {
                        end = mid-1;
                    }
                    else if(matrix[i][mid] < target) {
                        begin = mid+1;
                    }
                    if(matrix[i][mid]==target)
                        return true;
                }
            }
        }
        return false;
    }
};

33. 搜索旋转排序数组

二分法，稍微区分了一下左侧有序还是右侧有序

cpp 复制代码

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

153. 寻找旋转排序数组中的最小值

cpp 复制代码

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

98. 验证二叉搜索树

通过中序遍历，得到有序的数组，在判断数组是否严格递增

cpp 复制代码

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void traversal(TreeNode* root,vector<int>& res) {
        if(root==NULL)
            return;
        if(root->left)
            traversal(root->left,res);
        res.push_back(root->val);
        if(root->right)
            traversal(root->right,res);
    }
    bool isValidBST(TreeNode* root) {
        if(!root)
            return true;
        vector<int> res;
        traversal(root,res);
        for(int i=1;i<res.size();i++) {
            if(res[i] <= res[i-1])
                return false;
        }
        return true;
    }
};

118. 杨辉三角

res即dp数组，寻找res $i$ $j$ 的更新规律

cpp 复制代码

class Solution {
public:
    vector<vector<int>> generate(int numRows) {
        vector<vector<int>> res(numRows);
        for(int i=0;i<numRows;i++) {
            res[i].resize(i+1);
            res[i][0] = res[i][i] = 1;
            for(int j=1;j<i;j++) {
                res[i][j] = res[i-1][j]+res[i-1][j-1];
            }
        }
        return res;
    }
};