5.12

226. 翻转二叉树

/**
 * 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
 * @return {TreeNode}
 */
var invertTree = function(root) {
    if (root === null) {
        return root
    }
    const tmp = root.left
    root.left = root.right
    root.right = tmp
    invertTree(root.left)
    invertTree(root.right)
    return root
};

189 轮转数组

/**
 * @param {number[]} nums
 * @param {number} k
 * @return {void} Do not return anything, modify nums in-place instead.
 */
 // 用额外数组存即可 (i + k)% nums.length
var rotate = function(nums, k) {
    let len = nums.length
    let list = new Array(len).fill(0)
    for (let i = 0; i < len; i++) {
        list[(i + k) % len] = nums[i]
    }
  // 这里为什么要额外处理一次
    for (let i = 0; i < len; ++i) {
        nums[i] = list[i];
    }
    return nums
};

624，给表达式添加运算符

var addOperators = function(num, target) {
    const n = num.length;
    const ans = [];
    let expr = [];
    const backtrack = (expr, i, res, mul) => {
        if (i === n) {
            // 终止条件
            if (res === target) {
                ans.push(expr.join(''));
            }
            return;
        }
        const signIndex = expr.length;
        if (i > 0) {
            expr.push('');
        }
        let val = 0;
        // 枚举截取的数字长度（取多少位），注意数字可以是单个 0 但不能有前导零
        for (let j = i; j < n && (j === i || num[i] !== '0'); ++j) {
            val = val * 10 + num[j].charCodeAt() - '0'.charCodeAt();
            expr.push(num[j]);
            if (i === 0) { // 表达式开头不能添加符号
                backtrack(expr, j + 1, val, val);
            } else { // 枚举符号
                expr[signIndex] = '+';
                backtrack(expr, j + 1, res + val, val);
                expr[signIndex] = '-';
                backtrack(expr, j + 1, res - val, -val);
                expr[signIndex] = '*';
                backtrack(expr, j + 1, res - mul + mul * val, mul * val);
            }
        }
        expr = expr.splice(signIndex, expr.length - signIndex)
    }
    backtrack(expr, 0, 0, 0);
    return ans;
}

647. 回文子串

var countSubstrings = function(s) {
    const n = s.length;
    let ans = 0;
    for (let i = 0; i < 2 * n - 1; ++i) {
        let l = i / 2, r = i / 2 + i % 2;
        while (l >= 0 && r < n && s.charAt(l) == s.charAt(r)) {
            --l;
            ++r;
            ++ans;
        }
    }
    return ans;
};

12. 整数转罗马数字