Day 77, #31, Next Permutation

Next Permutation
Implement next permutation, which rearranges numbers into the lexicographically next greater permutation of numbers.
If such arrangement is not possible, it must rearrange it as the lowest possible order (ie, sorted in ascending order).
The replacement must be in-place, do not allocate extra memory.
Here are some examples. Inputs are in the left-hand column and its corresponding outputs are in the right-hand column.
1,2,3 → 1,3,2
3,2,1 → 1,2,3
1,1,5 → 1,5,1
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Solution #1
Find the last peak number i, swap [i - 1] and the number that follows [i - 1] in ascending order, then reverse all numbers from peak to end

class Solution {
public:
    void nextPermutation(vector<int> &num) {
        if (num.size() == 0 || num.size() == 1) return;
        int peak = -1;
        int swapIndex = 0;
        
        for (int i = 0; i < num.size(); i++) {
            if (i + 1 < num.size() && num[i] < num[i + 1]) {
                peak = i + 1;
                swapIndex = peak;
            }else if (peak != -1) {
                if (num[i] > num[peak - 1]) {
                    swapIndex = i;
                }
            }    
        }
        
        if (peak == -1) {
            reverse(num.begin(),num.end());
        }else {
            int temp = num[swapIndex];
            num[swapIndex] = num[peak - 1];
            num[peak - 1] = temp;
            
            reverse(num.begin() + peak,num.end());
        }
    }
};

Java, updated on Aug-6th-2018
6 9 7 3 2 -> 7 2 3 6 9 可以看出规律

注意数字有重复的时候，比如2 3 1 3 3 -> 2 3 3 1 3. 所有第36行对比时要加等号

class Solution {
    public void nextPermutation(int[] nums) {
        int drop = getFirstDrop(nums);
        if (drop == -1) {
            reverseArray(nums, 0);
            return;
        }
        
        int nextInOrder = getNextInOrder(nums, drop);
        swap(nums, drop, nextInOrder);
        reverseArray(nums, drop + 1);
    }
    
    private void reverseArray(int[] nums, int index) {
        int right = nums.length - 1;
        
        while (index < right) {
            swap(nums, index, right);    
            index++;
            right--;
        }
    }
    
    private void swap(int[] nums, int left, int right) {
        int temp = nums[left];
        nums[left] = nums[right];
        nums[right] = temp;
    }
    
    private int getNextInOrder(int[] nums, int drop) {
        
        int index = drop + 1;
        int min = nums[index];
        
        for (int i = index; i < nums.length; i++) {
            if (min >= nums[i] && nums[i] > nums[drop]) { // 注意“>=”
                min = nums[i];
                index = i;
            }
        }
        
        return index;
    }
    
    private int getFirstDrop(int[] nums) {
        
        for (int i = nums.length - 2; i >= 0; i--) {
            if (nums[i] < nums[i + 1]) {
                return i;
            }
        }
        
        return -1;
    }
}