Contest 153

2019-09-14 23:16:25

总体感受：节奏尚可，难度尚可。然而还是卡在了最后一题，完全没有想到使用dp。

注意点：

1）Contest的最后一题往往是DP，Graph，这个敏感度需要提高；

2）DP的题目还是远远不够熟练。

• 801. Minimum Swaps To Make Sequences Increasing

问题描述：

问题求解：

keep[i] : 保留i位置的数字，使得[0, i]两个数组递增的最小交换次数。

swap[i] : 交换i位置的数字，使得[0, i]两个数组递增的最小交换次数。

    public int minSwap(int[] A, int[] B) {
        int n = A.length;
        int[] keep = new int[n];
        int[] swap = new int[n];
        Arrays.fill(keep, n);
        Arrays.fill(swap, n);
        keep[0] = 0;
        swap[0] = 1;
        for (int i = 1; i < n; i++) {
            if (A[i] > A[i - 1] && B[i] > B[i - 1]) keep[i] = Math.min(keep[i], keep[i - 1]);
            if (A[i] > B[i - 1] && B[i] > A[i - 1]) keep[i] = Math.min(keep[i], swap[i - 1]);
            if (B[i] > A[i - 1] && A[i] > B[i - 1]) swap[i] = Math.min(swap[i], keep[i - 1] + 1);
            if (B[i] > B[i - 1] && A[i] > A[i - 1]) swap[i] = Math.min(swap[i], swap[i - 1] + 1);
        }
        return Math.min(keep[n - 1], swap[n - 1]);
    }

　　

• 1187. Make Array Strictly Increasing

问题描述：

问题求解：

    public int makeArrayIncreasing(int[] arr1, int[] arr2) {
        int n = arr1.length;
        int[] keep = new int[n];

        Set<Integer> set = new TreeSet<>();
        for (int num : arr2) set.add(num);
        int m = set.size();
        int[] sorted = new int[m];
        int idx = 0;
        for (int num : set) sorted[idx++] = num;
        int[] swap = new int[m];

        Arrays.fill(keep, n + 1);
        keep[0] = 0;
        for (int i = 0; i < m; i++) swap[i] = 1;
        for (int i = 1; i < n; i++) {
            if (arr1[i] > arr1[i - 1]) keep[i] = keep[i - 1];
            int cur_min = n;
            for (int j = 0; j < m; j++) {
                int cur_swap = n + 1;
                if (arr1[i] > sorted[j]) keep[i] = Math.min(keep[i], swap[j]);
                int temp = cur_min;
                cur_min = Math.min(cur_min, swap[j]);
                if (sorted[j] > arr1[i - 1]) cur_swap = Math.min(cur_swap, keep[i - 1] + 1);
                cur_swap = Math.min(cur_swap, temp + 1);
                swap[j] = cur_swap;
            }
        }
        int res = Math.min(n + 1, keep[n - 1]);
        for (int i = 0; i < m; i++) res = Math.min(res, swap[i]);
        return res > n ? -1 : res;
    }