988. 从叶结点开始的最小字符串

C代码：DFS

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
 // 叶子节点
 // 每一层用一个pathTop、遇到叶子节点就判断一次；建立一个ans字符串，每次叶子节点与之比较
 // 建一个全局的数组，最后再reverse

#define MAX_LEN 8501

char* tmp;    // reverse接收用, 末尾加'\0'
char* path;   // 全局路径, 用pathTop维系
char* ans;    // strcat自动加'\0'

char* reverse(int pathTop) {
    for (int i = 0; i < pathTop; ++i) {
        tmp[i] = path[pathTop - i -1];
    }
    tmp[pathTop] = '\0';
    return tmp;
}

void dfs(struct TreeNode* root, int pathTop) {
    if (root == NULL) {
        return;
    }
    path[pathTop++] = root->val + 'a';  // 先序遍历
    if (root->left == NULL && root->right == NULL) {
        char* tmp = reverse(pathTop);
        if (strlen(ans) == 0 || strcmp(ans, tmp) > 0) {  // ans 大于 tmp 返回 >0
            ans[0] = '\0';
            strcat(ans, tmp);
        }
    }
    dfs(root->left, pathTop);
    dfs(root->right, pathTop);
}

char * smallestFromLeaf(struct TreeNode* root){
    path = (char*)malloc(sizeof(char) * MAX_LEN);
    tmp = (char*)malloc(sizeof(char) * MAX_LEN);
    ans = (char*)malloc(sizeof(char) * MAX_LEN);   
    ans[0] = '\0';
    dfs(root, 0);
    return ans;
}