110. 平衡二叉树

function isBalanced(root: TreeNode | null): boolean {
    if (root === null) {
        return true;
    };
    const dfs = (node: TreeNode | null) => {
        if (node === null) {
            return 0;
        }
        const leftDepth = dfs(node.left);
        if (leftDepth === -1) {
            return -1;
        }
        const rightDepth = dfs(node.right);
        if (rightDepth === -1) {
            return -1;
        }
        if (Math.abs(leftDepth - rightDepth) > 1) {
            return -1;
        } else {
            return 1 + Math.max(leftDepth, rightDepth)
        }
    };
    return dfs(root) !== -1;
};

257. 二叉树的所有路径

function binaryTreePaths(root: TreeNode | null): string[] {
    let res = [];
    const dfs = (node: TreeNode | null, path: string) => {
        if (node.left === null && node.right === null) {
            path += node.val;
            res.push(path);
            return;
        };
        path += node.val + '->';
        node.left && dfs(node.left, path);
        node.right && dfs(node.right, path);
    };
    dfs(root, '');
    return res;
};

404. 左叶子之和

递归法

function sumOfLeftLeaves(root: TreeNode | null): number {
    if (root.left === null && root.right === null) {
    return 0;
    }
    let res = 0;
    const dfs = (node: TreeNode | null) => {
    if (node === null) {
    return;
    }
    if (node.left && node.left.left === null && node.left.right === null) {
    res += node.left.val;
    }
    node.left && dfs(node.left);
    node.right && dfs(node.right);
    };
    dfs(root);
    return res;
};

迭代法

function sumOfLeftLeaves(root: TreeNode | null): number {
    if (root.left === null && root.right === null) {
        return 0;
    }
    let res = 0;
    let queue = [root];
    while (queue.length) {
        const node = queue.shift();
        if (node.left !== null && node.left.left === null && node.left.right === null) {
            res += node.left.val;
        }
        node.left && queue.push(node.left);
        node.right && queue.push(node.right)
    }
    return res;
};