328. 奇偶連結串列

Tag: 連結串列

class Solution {
public:
    ListNode *oddEvenList(ListNode *head) {
        if (head == nullptr || head->next == nullptr) {
            return head;
        }
        ListNode *odd_h = head, *even_h = head->next;
        ListNode *p = odd_h, *q = even_h, *prev = nullptr;
        while (p) {
            auto nextP = q ? q->next : nullptr;
            auto nextQ = nextP ? nextP->next : nullptr;
            p->next = nextP;
            if (q) {
                q->next = nextQ;
            }
            prev = p;
            p = p->next;
            q = q ? q->next : nullptr;
        }
        prev->next = even_h;
        return odd_h;
    }
};
 

329. 矩陣中的最長遞增路徑

Tag: 動態規劃

class Solution {
public:
    int longestIncreasingPath(vector<vector<int>>& matrix) {
        vector<vector<int>> dirs = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}};
        int m = matrix.size(), n = matrix[0].size();
        vector<vector<int>> tmp;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                tmp.push_back({matrix[i][j], i, j});
            }
        }
        auto myCmpF = [](const vector<int>& a, const vector<int>& b) {
            return a[0] > b[0];
        };
        sort(tmp.begin(), tmp.end(), myCmpF);
        int res = 0;
        vector<vector<int>> dp(m, vector<int>(n, 1));
        for (auto& item : tmp) {
            int val = item[0], i = item[1], j = item[2];
            for (auto& dir : dirs) {
                int nextI = i + dir[0], nextJ = j + dir[1];
                if (nextI >= 0 && nextI < m && nextJ >= 0 && nextJ < n &&
                    matrix[nextI][nextJ] > val) {
                    dp[i][j] = max(dp[i][j], dp[nextI][nextJ] + 1);
                }
            }
            res = max(res, dp[i][j]);
        }
        return res;
    }
};
 

331. 驗證二叉樹的前序序列化

Tag: 二叉樹 棧

class Solution {
public:
    bool isValidSerialization(string preorder) {
        int n = preorder.size();
        int i = 0;
        stack<int> stk;
        stk.push(1);
        while (i < n) {
            if (stk.empty()) {
                return false;
            }
            if (preorder[i] == ',') {
                i++;
            } else if (preorder[i] == '#') {
                stk.top() -= 1;
                if (stk.top() == 0) {
                    stk.pop();
                }
                i++;
            } else {
                while (i < n && preorder[i] != ',') {
                    i++;
                }
                stk.top() -= 1;
                if (stk.top() == 0) {
                    stk.pop();
                }
                stk.push(2);
            }
        }
        return stk.empty();
    }
};
 

class Solution {
public:
    bool isValidSerialization(string preorder) {
        int n = preorder.size();
        int i = 0;
        int slots = 1;
        while (i < n) {
            if (slots == 0) {
                return false;
            }
            if (preorder[i] == ',') {
                i++;
            } else if (preorder[i] == '#') {
                slots--;
                i++;
            } else {
                while (i < n && preorder[i] != ',') {
                    i++;
                }
                slots = slots - 1 + 2;
            }
        }
        return slots == 0;
    }
};

334. 遞增的三元子序列

Tag: 貪心

class Solution {
public:
    bool increasingTriplet(vector<int>& nums) {
        int small = INT32_MAX, mid = INT32_MAX;
        for (int n : nums) {
            if (n <= small) {
                small = n;
            } else if (n <= mid) {
                mid = n;
            } else if (n > mid) {
                return true;
            }
        }
        return false;
    }
};

337. 打家劫舍 III

Tag: 動態規劃

class Solution {
private:
    unordered_map<TreeNode*, int> memo;

public:
    int rob(TreeNode* root) {
        if (root == nullptr) {
            return 0;
        }
        if (memo.find(root) != memo.end()) {
            return memo[root];
        }
        int robRoot = root->val;
        if (root->left) {
            robRoot += rob(root->left->left) + rob(root->left->right);
        }
        if (root->right) {
            robRoot += rob(root->right->left) + rob(root->right->right);
        }
        int noRobRoot = rob(root->left) + rob(root->right);
        int ans = max(robRoot, noRobRoot);
        memo[root] = ans;
        return ans;
    }
};

338. 位元位計數

Tag: 動態規劃

class Solution {
public:
    vector<int> countBits(int n) {
        vector<int> res(n + 1, 0);
        for (int i = 1; i < n + 1; i++) {
            if ((i & 1) == 0) {
                //偶數
                res[i] = res[i >> 1];
            } else {
                //奇數
                res[i] = res[i - 1] + 1;
            }
        }
        return res;
    }
};

class Solution {
public:
    vector<int> countBits(int n) {
        vector<int> dp(n + 1, 0);
        int highBit = 0;
        for (int i = 1; i <= n; i++) {
            if ((i & (i - 1)) == 0) {
                highBit = i;
            }
            dp[i] = dp[i - highBit] + 1;
        }
        return dp;
    }
};

343. 整數拆分

Tag: 動態規劃

class Solution {
public:
    int integerBreak(int n) {
        vector<int> dp(n + 1, 0);
        dp[2] = 1;
        for (int i = 3; i <= n; i++) {
            int tmp = 0;
            for (int j = 2; j < i; j++) {
                tmp = max(max(tmp, j * (i - j)), j * dp[i - j]);
            }
            dp[i] = tmp;
        }
        return dp[n];
    }
};