# LeetCode Weekly Contest 258解题报告

【 NO.1 反转单词前缀】

class Solution {
public String reversePrefix(String word, char ch) {
int index = word.indexOf(ch);
return new StringBuffer(word.substring(0, index + 1)).reverse().toString() +
word.substring(index + 1);
}
}

【 NO.2 可互换矩形的组数】

class Solution {
static class Frac {
int den;
int num;

``````   public static int gcd(int a, int b) {
return b == 0 ? a : gcd(b, a % b);
}

public Frac(int num, int den) {
int g = gcd(num, den);
this.num = num / g;
this.den = den / g;
}

@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Frac frac = (Frac) o;
return den == frac.den && num == frac.num;
}

@Override
public int hashCode() {
return Objects.hash(num, den);
}
``````

}

public long interchangeableRectangles(int[][] rectangles) {
Map<Frac, Integer> count = new HashMap<>();
for (var rec : rectangles) {
Frac f = new Frac(rec[0], rec[1]);
count.put(f, count.getOrDefault(f, 0) + 1);
}
long res = 0;
for (var k : count.entrySet()) {
int v = k.getValue();
res += (long) v * (v - 1) / 2;
}
return res;
}
}

【 NO.3 两个回文子序列长度的最大乘积】

class Solution {
public int maxProduct(String s) {
int len = s.length();
int res = 0;
int[] mem = new int[1 << len];
Arrays.fill(mem, -1);
for (int i = 0; i < (1 << len); i++) {
for (int j = 0; j < (1 << len); j++) {
if ((i & j) > 0) {
continue;
}
res = Math.max(res, length(s, i, mem) * length(s, j, mem));
}
}
return res;
}

private int length(String s, int bitset, int[] mem) {
if (mem[bitset] >= 0) {
return mem[bitset];
}
mem[bitset] = 0;
for (int i = 0, j = s.length() - 1; i <= j; i++, j–) {
while (i <= j && (bitset & (1 << i)) == 0) i++;
while (i <= j && (bitset & (1 << j)) == 0) j–;
if (!(i <= j && (bitset & (1 << i)) != 0 && (bitset & (1 << j)) != 0)) {
break;
}
if (s.charAt(i) == s.charAt(j)) {
mem[bitset] += i == j ? 1 : 2;
} else {
mem[bitset] = 0;
break;
}
}
return mem[bitset];
}
}

【 NO.4 每棵子树内缺失的最小基因值】

DFS 合并 Set 即可。但是有两个优化很重要：

1. 假如子树中缺失的最大的是 x, 那么枚举查找当前树缺失的只需要从 x 开始即可，而不是 1

2. 合并 Set 时由小 Set 合并到大 Set 中

class Solution {
public int[] smallestMissingValueSubtree(int[] parents, int[] nums) {
Map<Integer, List> children = new HashMap<>();
for (int i = 1; i < parents.length; i++) {
if (!children.containsKey(parents[i])) {
children.put(parents[i], new ArrayList<>());
}
}
int[] ans = new int[parents.length];
dfs(0, children, nums, ans);
return ans;
}

private Set dfs(int cur, Map<Integer, List> children, int[] nums, int[] ans) {
Set set = new HashSet<>();
if (!children.containsKey(cur)) {
ans[cur] = nums[cur] == 1 ? 2 : 1;
return set;
}
var child = children.get(cur);
int start = 1;
for (var c : child) {
var r = dfs(c, children, nums, ans);
if (r.size() > set.size()) {
Set tmp = r;
r = set;
set = tmp;
}