LeetCode与《代码随想录》二叉树篇：做题笔记与总结-JavaScript版

Posted 2023-03-10 karshey

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文章目录

代码随想录

代码随想录
 代码随想录CSDN官方

前、中、后指的都是根的位置。

144. 二叉树的前序遍历

https://leetcode.cn/problems/binary-tree-preorder-traversal/

var preorderTraversal = function (root) 
    // 前序遍历：中 左 右
    let ans = []

    const dfs = function (root) 
        if (root === null) return;
        ans.push(root.val);
        dfs(root.left);
        dfs(root.right);
    

    dfs(root);

    return ans;
;

94. 二叉树的中序遍历

https://leetcode.cn/problems/binary-tree-inorder-traversal/

var inorderTraversal = function (root) 
    let ans = []
    const dfs = function (root) 
        if (root === null) return;
        // 中序:左中右
        dfs(root.left);
        ans.push(root.val);
        dfs(root.right);
    
    dfs(root);
    return ans;
;

145. 二叉树的后序遍历

https://leetcode.cn/problems/binary-tree-postorder-traversal/

var postorderTraversal = function (root) 
    let ans = []
    const dfs = function (root) 
        if (root === null) return;
        // 后序:左右中
        dfs(root.left);
        dfs(root.right);
        ans.push(root.val);
    
    dfs(root);
    return ans;
;

102.二叉树的层序遍历

https://leetcode.cn/problems/binary-tree-level-order-traversal/

var levelOrder = function (root) 
    let ans = [], queue = []
    if (root === null) return ans;
    queue.push(root);
    while (queue.length) 
        let len = queue.length;
        let anss = [];
        for (let i = 0; i < len; i++) 
            let node = queue.shift();
            anss.push(node.val);
            node.left && queue.push(node.left);
            node.right && queue.push(node.right);
        
        ans.push(anss);
    
    return ans;
;

226.翻转二叉树

https://leetcode.cn/problems/invert-binary-tree/

var invertTree = function (root) 
    const dfs = function (root) 
        if (root === null) return;
        dfs(root.left);
        dfs(root.right);
        [root.left, root.right] = [root.right, root.left];
    
    dfs(root);
    return root;
;

101. 对称二叉树

https://leetcode.cn/problems/symmetric-tree/

var isSymmetric = function (root) 
    let ans = true;
    if (root === null) return ans;
    if (root.left && root.right === null) return false;
    if (root.right && root.left === null) return false;
    const dfs = function (l, r) 
        if (!l && !r) return;
        if (l && !r) 
            ans = false; return;
        
        if (!l && r) 
            ans = false; return;
        
        if (l.val !== r.val) 
            ans = false;
            return;
        
        dfs(l.left, r.right);
        dfs(l.right, r.left);
    

    dfs(root.left, root.right);
    return ans;
;

104.二叉树的最大深度

https://leetcode.cn/problems/maximum-depth-of-binary-tree/

var maxDepth = function (root) 
    let ans = 0;
    if (!root) return ans;
    let q = [];
    q.push(root);
    while (q.length) 
        ans++;
        let len = q.length;
        for (let i = 0; i < len; i++) 
            let node = q.shift();
            node.left && q.push(node.left);
            node.right && q.push(node.right);
        
    
    return ans;
;

111.二叉树的最小深度

https://leetcode.cn/problems/minimum-depth-of-binary-tree/

var minDepth = function (root) 
    // 若一个节点没有叶子节点，则结束
    let ans = 0;
    if (!root) return ans;
    let q = [];
    q.push(root);
    while (q.length) 
        ans++;
        let len = q.length;
        for (let i = 0; i < len; i++) 
            let node = q.shift();
            if (!node.left && !node.right) return ans;
            node.left && q.push(node.left);
            node.right && q.push(node.right);
        
    
;

222.完全二叉树的节点个数

https://leetcode.cn/problems/count-complete-tree-nodes/

注意：要利用完全二叉树的性质。

var countNodes = function (root) 
    if (!root) return 0;
    let l = root.left, r = root.right;
    let ll = 0, rr = 0;
    while (l) 
        ll++; l = l.left;
    
    while (r) 
        rr++; r = r.right;
    
    if (ll === rr) 
        return Math.pow(2, ll + 1) - 1;
    
    else 
        return countNodes(root.left) + countNodes(root.right) + 1;
    
;

110.平衡二叉树

https://leetcode.cn/problems/balanced-binary-tree/

var isBalanced = function (root) 
    var getHeight = function (root) 
        if (!root) return 0;
        let l = getHeight(root.left);
        if (l === -1) return -1;
        let r = getHeight(root.right);
        if (r === -1) return -1;

        if (Math.abs(l - r) > 1) return -1;
        else return Math.max(l, r) + 1;
    

    let ans = getHeight(root);
    if (ans === -1) return false;
    else return true;
;

257. 二叉树的所有路径

https://leetcode.cn/problems/binary-tree-paths/

var binaryTreePaths = function (root) 
    let ans = [], anss = [];
    var dfs = function (root) 
        anss.push(root.val);
        root.left && dfs(root.left);
        root.right && dfs(root.right);
        if (!root.left && !root.right) 
            ans.push(anss.join('->'));
        
        anss.pop();
    

    dfs(root);
    return ans;
;

404.左叶子之和

https://leetcode.cn/problems/sum-of-left-leaves/

var sumOfLeftLeaves = function (root) 
    let ans = 0;
    var dfs = function (root, flag) 
        if (!root) return;
        root.left && dfs(root.left, 1);
        root.right && dfs(root.right, 0);
        if (flag === 1 && !root.left && !root.right) 
            ans += root.val;
        
    

    dfs(root);
    return ans;
;

513.找树左下角的值

https://leetcode.cn/problems/find-bottom-left-tree-value/

var findBottomLeftValue = function (root) 
    // 记录bfs每一层的第一个
    let ans = root.val, q = [];
    q.push(root);
    while (q.length) 
        let len = q.length;
        for (let i = 0; i < len; i++) 
            let node = q.shift();
            if (!i) ans = node.val;
            node.left && q.push(node.left);
            node.right && q.push(node.right);
        
    
    return ans;
;

112. 路径总和

https://leetcode.cn/problems/path-sum/

var hasPathSum = function (root, targetSum) 
    let ans = 0, flag = false;
    var dfs = function (root) 
        if (root.left) 
            ans += root.left.val;
            dfs(root.left);
            ans -= root.left.val;
        
        if (root.right) 
            ans += root.right.val;
            dfs(root.right);
            ans -= root.right.val;
        
        if (!root.left && !root.right) 
            if (ans === targetSum) flag = true;
        
    
    if (root) 
        ans += root.val;
        dfs(root);
    


    return flag;
;

106.从中序与后序遍历序列构造二叉树

https://leetcode.cn/problems/construct-binary-tree-from-inorder-and-postorder-traversal/

var buildTree = function (inorder, postorder) 
    if (!inorder.length) return null;
    let rootVal = postorder.pop();
    let rootIndex = inorder.indexOf(rootVal);
    let node = new TreeNode(rootVal);
    node.left = buildTree(inorder.slice(0, rootIndex), postorder.slice(0, rootIndex));
    node.right = buildTree(inorder.slice(rootIndex + 1), postorder.slice(rootIndex));
    return node;
;

105. 从前序与中序遍历序列构造二叉树

https://leetcode.cn/problems/construct-binary-tree-from-preorder-and-inorder-traversal/

var buildTree = function (preorder, inorder) 
    if (!preorder.length) return null;
    let rootVal = preorder.shift();
    let rootIndex = inorder.indexOf(rootVal);
    let node = new TreeNode(rootVal);
    node.left = buildTree(preorder.slice(0, rootIndex), inorder.slice(0, rootIndex));
    node.right = buildTree(preorder.slice(rootIndex), inorder.slice(rootIndex + 1));
    return node;
;

654.最大二叉树

https://leetcode.cn/problems/maximum-binary-tree/

var constructMaximumBinaryTree = function (nums) 
    // 数组的左右边界
    var dfs = function (l, r) 
        if (l > r) return null;
        let maxVal = -1, maxIndex = -1;
        for (let i = l; i <= r; i++) 
            if (nums[i] > maxVal) 
                maxVal = nums[i];
                maxIndex = i;
            
        
        const node = new TreeNode(maxVal);
        node.left = dfs(l, maxIndex - 1);
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