二叉树 -左叶子之和

404. 左叶子之和

方法一：递归法

/**
 * 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 {number}
 */
var sumOfLeftLeaves = function (root) {
    // 采用后序遍历 递归遍历
    // 1. 确定递归函数参数
    const nodesSum = function (node) {
        // 2. 确定终止条件
        if (node === null) {
            return 0;
        }
        let leftValue = nodesSum(node.left);
        let rightValue = nodesSum(node.right);
        // 3. 确定单层递归逻辑
        let midValue = 0;
        if (node.left && node.left.left === null && node.left.right === null) {
            midValue = node.left.val;
        }
        let sum = midValue + leftValue + rightValue;
        return sum;
    }
    return nodesSum(root);
}

方法二：迭代法

/**
 * 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 {number}
 */
var sumOfLeftLeaves = function (root) {
    if (!root) {
        return 0;
    }
    let queue = [];
    let sum = 0;
    queue.push(root);
    while (queue.length) {
        let node = queue.shift();
        if (node.left !=null && node.left.left === null && node.left.right === null) {
            sum += node.left.val;
        }
        node.left && queue.push(node.left);
        node.right && queue.push(node.right);
    }
    return sum;
}