5.16

443. 压缩字符串

/**
 * @param {character[]} chars
 * @return {number}
 */
// 先压缩，再返回字符串长度
// 为什么不对
var compress = function(chars) {
    let res = []
    for (let i = 0; i < chars.length; i++) {
        res.push(chars[i])
        let j = i + 1
        while(chars[i] === chars[j]) {
            j++
        }
        j - i > 1 && res.push(String(j - i))
        i = j - 1
    }
    return res
};

剑指 Offer 27. 二叉树的镜像

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
 // 本质每一层做个反转，层次遍历reverse
var mirrorTree = function(root) {
    let q = [root]
    if (root === null) return null
    while(q.length) {
        for (let i = 0; i < q.length; i++) {
            let node = q.shift()
            if (node.left) q.push(node.left)
            if (node.right) q.push(node.right)
            let temp = node.left;
            node.left = node.right;
            node.right = temp;
        }
    }
    return root
};

1312. 让字符串成为回文串的最少插入次数

16. 最接近的三数之和

/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number}
 */
// 头尾双指针，如果发现大于，则右指针缩小
var threeSumClosest = function (nums, target) {
  nums.sort((a, b) => a - b);
  let res = nums[0] + nums[1] + nums[nums.length - 1];
  for (let i = 0; i < nums.length - 2; i++) {
    const n1 = nums[i];
    let l = i + 1;
    let r = nums.length - 1;
    while (l < r) {
      const n2 = nums[l];
      const n3 = nums[r];
      const sum = n1 + n2 + n3;
      if (sum > target) {
        r--;
      } else {
        l++;
      }
      if (Math.abs(sum - target) < Math.abs(res - target)) {
        res = sum;
      }
    }
  }
  return res;
};