栈、队列、优先队列笔记

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20. Valid Parentheses

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150. Evaluate Reverse Polish Notation

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71. Simplify Path

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144. Binary Tree Preorder Traversal

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94. Binary Tree Inorder Traversal

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145. Binary Tree Postorder Traversal

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341. Flatten Nested List Iterator

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102. Binary Tree Level Order Traversal

// 此代码可以再更改，参看视频内容
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> result;
        if(!root)
            return result;
        int toBePrinted = 1;
        int nextLevel = 0;
        TreeNode* pCur = root;
        queue q;
        q.push(root);
        vector<int> v;
        while(!q.empty()){
            pCur = q.front();
            q.pop();
            v.push_back(pCur->val);
            --toBePrinted;
            if(pCur->left){
                q.push(pCur->left);
                ++nextLevel;
            }
            if(pCur->right){
                q.push(pCur->right);
                ++nextLevel;
            }
            if(!toBePrinted){
                result.push_back(v);
                v.clear();
                toBePrinted = nextLevel;
                nextLevel = 0;
            }
        }
        return result;
    }
};

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107. Binary Tree Level Order Traversal II

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103. Binary Tree Zigzag Level Order Traversal

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199. Binary Tree Right Side View

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BFS和图的最短路径

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279. Perfect Squares
之前用动态规划做过
用图的最短路径解决一下

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127. Word Ladder

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126. Word Ladder II

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优先队列的底层实现：堆
堆的底层实现：白板编程
priority_queue<int> 默认最大堆
priority_queue<int, vector<int>, greater<int>> // 最小堆
priority_queue<int, vector<int>, compare>

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347. Top K Frequent Elements

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23. Merge k Sorted Lists

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