LeetCode 分类之二叉树剪枝相关

814. 二叉树剪枝

814. 二叉树剪枝

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var pruneTree = function(root) {
    // const dfs = (root) => {
    //     if (root == null) return 0;
    //     const l = dfs(root.left);
    //     const r = dfs(root.right);

    //     if (l == 0) root.left = null;
    //     if (r == 0) root.right = null;
    //     return root.val + l + r;
    // }
    // let ans = new TreeNode(-1);
    // ans.left = root;

    // dfs(ans);
    // return ans.left;

    const dfs = (node) => {
        if (!node) return 0;

        const l = dfs(node.left);
        const r = dfs(node.right);

        if (l == 0) node.left = null;
        if (r == 0) node.right = null;
        return node.val + l + r;
    }
    let res = new TreeNode(-1);
    res.left = root;
    dfs(res);
    return res.left;
};

669. 修剪二叉搜索树

669. 修剪二叉搜索树

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} low
 * @param {number} high
 * @return {TreeNode}
 */
var trimBST = function(root, low, high) {

    if (!root) return null;
    if (root.val < low) {
        return trimBST(root.right, low, high);
    }
    if (root.val > high) {
        return trimBST(root.left, low, high);
    }
    root.left = trimBST(root.left, low, high);
    root.right = trimBST(root.right, low, high);
    return root;
};

1325. 删除给定值的叶子节点

1325. 删除给定值的叶子节点

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} target
 * @return {TreeNode}
 */
/* 
    删除指定值的叶子节点，其实就是遍历所有的叶子节点，然后判断是否需要删除；删除叶子节点也很简单，return null 让父节点接收即可。
    难点在于他这个删除操作是循环的，一直删到叶子结点不存在 target 为止
    一个节点要在后序位置接收左右子树的返回值，才能知道自己的叶子节点是否都被删掉了，以此判断自己是不是变成了叶子节点。
*/
var removeLeafNodes = function(root, target) {
    if (!root) return null;

    root.left = removeLeafNodes(root.left, target);
    root.right = removeLeafNodes(root.right, target);

    if (root.val === target && root.left == null && root.right == null) {
        return null;
    }
    return root;
};