算法练习（9）：牛客在线编程09 双指针

package jz.bm;

import jz.Interval;

import java.util.ArrayList;
import java.util.HashMap;

public class bm9 {
    /**
     * BM87 合并两个有序的数组
     */
    public void merge(int A[], int m, int B[], int n) {
        int i = m - 1, j = n - 1, k = m + n - 1;
        while (i >= 0 || j >= 0) {
            int a = i >= 0 ? A[i] : Integer.MIN_VALUE;
            int b = j >= 0 ? B[j] : Integer.MIN_VALUE;
            if (a > b) {
                A[k] = a;
                i--;
            } else {
                A[k] = b;
                j--;
            }
            k--;
        }
    }

    /**
     * BM88 判断是否为回文字符串
     */
    public boolean judge (String str) {
        if (str == null || str.equals("")) {
            return true;
        }
        for (int i = 0; i < str.length() / 2; i++) {
            if (str.charAt(i) != str.charAt(str.length() - 1 - i)) {
                return false;
            }
        }
        return true;
    }

    /**
     * BM89 合并区间
     */
    public ArrayList<Interval> merge(ArrayList<Interval> intervals) {
        intervals.sort((o1, o2) -> o1.start - o2.start);
        int pre = 0, cur = 1;
        while (cur < intervals.size()) {
            //无法合并
            if (intervals.get(pre).end < intervals.get(cur).start) {
                pre = cur;
                cur++;
            } else if (intervals.get(pre).end >= intervals.get(cur).start) { //可以合并
                intervals.get(cur).start = intervals.get(pre).start;
                intervals.get(cur).end = Math.max(intervals.get(pre).end, intervals.get(cur).end);
                intervals.remove(pre);
            }
        }
        return intervals;
    }

    /**
     * BM91 反转字符串
     */
    public String solve (String str) {
        StringBuilder stringBuilder = new StringBuilder();
        for (int i = str.length() - 1; i >= 0; i--) {
            stringBuilder.append(str.charAt(i));
        }
        return stringBuilder.toString();
    }

    /**
     * BM92 最长无重复子数组
     */
    public int maxLength (int[] arr) {
        //双指针 + hashmap去重
        if (arr.length == 0) {
            return 0;
        }
        int res = 0;
        int i = 0, j = 0;
        HashMap<Integer, Integer> map = new HashMap<>();
        while (j < arr.length) {
            //新加入是字符有重复，更新i
            if (map.containsKey(arr[j])) {
                i = Math.max(i, map.get(arr[j]) + 1);
            }
            map.put(arr[j], j);
            res = Math.max(res, j - i + 1);
            j++;
        }
        return res;
    }

    /**
     * BM93 盛水最多的容器
     */
    public int maxArea (int[] height) {
        //双指针 + 贪心算法
        if (height.length <= 1) {
            return 0;
        }
        int i = 0, j = height.length - 1, res = 0;
        while (i < j) {
            res = Math.max(res, Math.min(height[i], height[j]) * (j - i));
            if (height[i] < height[j]) {
                i++;
            } else {
                j--;
            }
        }
        return res;
    }
}